The scenario is just about impossible. That is to say it could have sentient inhabitants but for them to have roughly the same technology level as us is just about impossible - unless they were colonized by us or one of our creations or descendants at some earlier stage (i.e. this scenario happens a century or two into our future).

Why do I say that? Well the first microscopic multicellular life that lead to humans arose about 500 million years ago, but our planet is over 4.5 billion years old. Even if you had a duplicate of Earth, formed at the same time as Earth, why would that happen exactly 4 billion years later? And why would it take 500 million years after that to reach the point where a technological species to evolve, instead of say 400 million or 600 million. What about the earlier evolution of photosynthetic life, which was necessary for an oxygen rich atmosphere - what if that was a few hundred million years earlier or later?

One of the most habitable places in our solar system is Europa’s ocean, in our own solar system. With an oxygen rich ocean, there may be some small chance of a civilization of non technological fish or octopuses or some such. If so, then even though it formed around the same time as Earth, the chance that any civilization there is just a few thousand or a few hundred thousand years old must be tiny. Chances are, either there is no civilization there, or it is millions of years old. That’s just in our own solar system.

Now add in that our star was by no means the first star that life could evolve around. Their star could have formed up to ten billion years ago. And now you are supposing that they are at the same technological level as us, i.e. their development so exactly paralleled ours that they are at the same level of technology we have in the 21st century, not the nineteenth century or the year 3000 or whatever. What’s the chance of that?

I know that it’s the assumption in many movie plots, but it requires a lot of “suspension of disbelief”. In the Star Trek universe they hypothesize “ancient humanoids” that seeded many planets with DNA so designed that they would all evolve to civilizations of humanoids with space faring technology at around the same time, to within a few centuries, 4.5 billion years later.

Ancient humanoid who in the Star Trek universe seeded many planets with life which independently evolved to humanoid lifeforms which then evolve space faring technology within a few centuries of each other 4.5 billion years later

That maybe works in fiction, but in reality, the idea is just absurd.

So they would be likely to be a millions of years old civilization. Either they have no technology (e.g. in an aquatic dolphin society, or even the likes of clever parrots, able to do some manipulation of objects but not to the extent we can), in which case it is a walk over for us, or they have millions of years old technology, in which case we have no chance unless they have just decided not to defend themselves.

This also leads to the question - why is the galaxy not filled already with extra terrestrials? Well if we find any extra terrestrials, they can’t possibly be expanding populations either, not exponentially expanding, because even doubling every thousand years, and travelling at 10% the speed of light average, they would fill the galaxy in its entirety well within a million years.

They would be here already and would have conquered Earth millions of years ago, if they wanted it, and would be in severe difficulty trying to find resources for all their colonists in the galaxy (doubling just every thousand years, you need all the resources in the observable universe to keep going for 180,000 years, so well before that time they would have mass starvation, wars, or in some other way their expansion would slow down).

Why wait for us to evolve before making use of Earth for their colonists?

I think myself that the reason for this is because any extraterrestrials who contemplate colonizing a galaxy realize that if they do this, soon the galaxy will be filled with their progeny, their creations, the evolved descendants of their biological and mechanical and cyborg replicators they create or give birth to, and the ones that are most aggressive and expansionist would win the race to fill the galaxy. What’s more, unless they have faster than light travel, you can’t have a civilization with shared values. Someone or some creature or machine a thousand light years away could evolve and develop strange ideas and philosophies - like ISIS perhaps but far worse, for a thousand years, before you learn about them.

Before colonizing the galaxy, if they do it at all, they would find a way around that. And I think we will also if far seeing. If not, well we probably won’t last long in space, in the fragile habitats and with technology more powerful than ICBMs available to just about anyone. I think we’ve made a good start with the Outer Space Treaty but need to be careful about how we continue. For these reasons I think we shouldn’t rush into colonization, that it’s neutral, can be harmful or good. But filling the galaxy especially - I don’t think that should be our goal.

If we did try to colonize the galaxy, then, yes, if there were any beings vulnerable to us in the galaxy, e.g. without technology, however advanced in the realm of ideas, they’ve had it. But the same also applies to us, or our descendants, they would never know when some scourge would appear from beyond the light speed horizon, suddenly approaching them at close to the speed of light, using technology they developed in isolation for thousands of years. If the movie makers made these movies on the premise that the monster ETs are ourselves, then yes, that would make a lot of sense, because then that would explain why they have the same or similar technology level to us. But if they are genuine ETs they almost certainly have to be stable, can’t be exponentially growing, and must have found a way through this dilemma of how to safely colonize or explore a galaxy.

And the safest way to do it at present, with our level of technology and our level of understanding of society and politics etc, is with robots. Whether there are other solutions that let us explore or even colonize parts of the galaxy without irreversibly trashing it and causing endless problems for ourselves and other beings in it - I don’t know. I hope so. But if there is, well there’s plenty of space for quadrillions of people in just a single solar system. There’s no need to fill the entire galaxy with our kind and if there are ETs I think it is obvious they have decided not to fill the galaxy, and to do that, they need a stable civilization, shared values, and communication as well, which I think means they occupy a geographically small area of the galaxy permanently, and only explorers elsewhere. Either that or they have immensely long lives which could permit shared values across an entire galaxy. Or warp drive, and very strong shared values?

The other possibility some suggest in solutions to the “Fermi paradox” is that technological civilizations galaxy wide never last more than a few centuries and then go extinct. That could lead to a galaxy filled with technological species at the same level of development as us. But you have to assume they all go extinct before they can colonize the galaxy, otherwise, it just needs a few descendants several light years away to not destroy themselves for them to fill the galaxy with continuous successive waves of colonists. And once that happens, how could they ever go extinct, except by being out competed by more advanced colonists? Even if most of them decide voluntarily to end their lives, or destroy themselves in warfare or whatever, it just needs one small group beyond the light speed barrier to survive, and the exponential growth will continue and soon refill all the vacated solar systems.

For that reason I don’t find this scenario plausible myself.

My solution is that yes, sometimes extra terrestrials do evolve to the point where they could fill the galaxy - but the ones that get that far are farsighted enough to realize the dangers involved in an uncontrolled expansion of themselves and their creations throughout the galaxy and the impossibility of continued exponential growth for ever, and they also have the cohesiveness and enough of shared values to make the decision not to do it. For them it would be as unthinkable as setting off a nuclear weapon in one of your own cities.

Either that or we are the first to evolve to this level of technology in our galaxy. If so then we need to take especial care as we could easily become the ones who trash the galaxy.

SO WHAT ABOUT THE SCENARIO WHERE WE COLONIZED THE PLANET OR OUR CREATIONS DID?

In that case, we are talking about some time in the near future as we haven’t colonized other planets yet. But if we did set up colonies with no precautions to prevent colonization from becoming a hazard for the galaxy long term - then they might well have similar levels of technology to us.

In that case, it just needs uneven technology development. For instance one of us develops self replicating nanotech or expertise in synthetic biology and the other doesn’t. Or one of us develops the ability to create and manipulate mini black holes and the other doesn’t. Then whether it might happen is a political question, and if one or the other is controlled by political extremists of some sort, then yes, they might do it.

Or in a less intentionally aggressive situation - they might just be overwhelmed by numbers and commercial interests. We set up home in their Oort cloud, and they don’t, or in smaller numbers, and we continually expand in population and they don’t, and once their Oort cloud and all the asteroids and planets in their solar system are converted into habitats for us, they wouldn’t last long after that. Not unless we decided as a matter of policy to leave them alone.

The same would happen as easily of course the other way around with them as the aggressors setting up home in our Oort cloud. And with nanotechnology, they might use replicating nanobots to convert our Oort cloud into habitats for them before occupying it, or vice versa, and with exponential population growth it might then happen very quickly.

I think this is a reason why we need to be cautious about colonizing the galaxy. That just through sheer weight of numbers if nothing else, and uneven technology development, the galaxy would get filled with expanding waves of creatures / creations that would vanquish anything that tries to stop them, with motives that may be obscure to us, and all eventually the result directly or indirectly of early colonization attempts by humans. The easiest way to prevent this is to explore the galaxy robotically, and to take care not to set up colonies if humans explore. There might be other solutions but we have to solve this first, in one way or another, before we think about colonizing the galaxy if we want to do it in a safe way for both ourselves and the galaxy.

For more on this see this section in my Case For Moon First

The exponential growth calculation I refer to is here: Why ET Populations Can't Continue To Expand For More Than A Few Millennia