It's possible to reuse some of the ISS modules. The thing is that most of them are getting to the end of their design life. The newest modules could be re-used and the Russians did talk about using them for a new space station in LEO. I don't know if that is going to happen.

In future maybe we will find the many tons of metal in space of value, but at present we don't have any manufacturing facilities to reuse them.

We have so little by way of reuse in space, that satellites are often abandoned just because they run out of fuel they need for positioning and station keeping, a bit like driving a car along a highway and when it runs out of fuel, you leave it by the highway and get your company to send you another car to continue the journey.

So, generally it's easier to just build a new spaceship than to repurpose and old one in space. The new one would have the latest technology and whatever you need for the mission.

I would imagine that if the ISS modules are reused, they'd be more likely to be used for a new space station in LEO as that's what they were designed for, and surely we will continue to have a use for that. But maybe not occupied by humans 24/7? And even then it might be easier to send up a new inflatable space station from Earth, if the Bigelow approach works. Because you have to factor in all the missions to reassemble it into a new space station too.

In principle if you had big enough rockets you could move the ISS to the Moon perhaps to the Earth Moon L1 position. You'd think it would work there though it would need more shielding to cope with solar storms.

But it's rather heavy, and if you did it with Soyuz rockets, it would take hundreds of missions probably. If you have a big enough rocket to do that easily, then you can also probably launch a new space station from Earth easily too. E.g. SLS or Falcon Heavy - are they best used for moving the ISS somewhere else, or just for launching new modules into space?

So, probably most of it will just be de-orbited. Some modules may be saved and repurposed as modules for a new station in LEO.

Another option would be to move it up into a higher orbit, so that it doesn't de-orbit right away, keep it safe for a few more decades or even longer if you put it in a high enough orbit. Could certainly do that, say as a museum or something, but that would be an expensive mission also.

It seems like a waste somehow, that hundred billion dollars and after a few decades, it all de-orbits and there is nothing left of it. But that's the way it is for all the stations in LEO so far and you can understand the logic, as it is not just a few thousand or million dollars to preserve it, but at least hundreds of millions, and you need a  very good reason to spend so much to preserve a space station for the future. And doesn't seem that they can come up with one, not so far anyway. It's valued for what it is now rather than as something for posterity like a monument.