Well, apart from anything else, much of the population of Tibet was nomadic. Then, anyone could be a monk, and in practice most families had a few of their children as monks or nuns. A large part of the population were monks. The Chinese make a big thing of this, but - it's hard to say at the same time that the monks were oppressors, and that anyone could be a monk and many were.
Then also, the "reincarnate lamas" whatever you think about them, were not born in particularly privileged families. The Dalai Lama for instance was a son of a small farmer.
And - many did own their own land. The monks were also supported to a large extent by generosity - as is the case in all the Buddhist countries. And the monasteries varied in size, some small communities might have a tiny monastery with just a few monks which they supported. It wasn't at all "imposed from above" but an expression of the religious views of the population - you can see this in other Buddhist countries, which also have monks and monasteries supported by the general public who often see it as a blessing, not an imposition, to support the monks. So no reason to suppose it was different in old Tibet.
Which is not to say at all that it was perfect, or that it didn't need to change. Surely it did, in many ways. Seems that it was a basically medieval society. But they were already changing. And as you can see from Bhutan, a similar country has been able to continue to modernise and change and adapt to the twentieth century without need for anyone else to force them to do it. The Dalai Lama himself was keen on change but had to leave Tibet soon after he "came of age" as you might say.
Probably the Chinese coming to Tibet to "liberate the oppressed" in a culture very different from China may have misunderstood the nature of the situation, and are not the most reliable historians for the situation in Tibet before their "reforms".
For some of the academic arguments back and forth on this topic, you can take a look at the wikipedia article which has many citations: Serfdom in Tibet controversy
But I haven't studied it as an academic myself :). So these are just general remarks. And - another thought - that for Buddhists then the most important thing is happiness rather than material advancement, as is clear in the case of Bhutan who have adopted the principle of Gross national happiness in Bhutan: the big idea from a tiny state that could change the world. Though one of the poorest nations in the world, it often ranks amongst one of the happiest.
So - it could also be that the Tibetans were happier back then than they were after the Chinese re-organizations / invasion or whatever you call it.
Whatever, it's not possible to roll back the clock, and Tibetans are not asking for a return to the old Tibetan systems. They are looking for a way forward instead, to some new future.