True, it's a sad situation for the Tibetans. It still is quite recent, I remember seeing footage on a TV documentary before. We can't really say that Tibet was prosperous under the Dalai Lama, it was quite feudal. Still, they were just chill, minding their own business, not causing much problem.
Yes. Tibet was not very prosperous under the Dalai Lama (at that point of time, China was not that prosperous either). But they were free to practice their indigenous culture and religion. The Chinese have attempted to wipe out all aspects of the native Tibetan culture ever since they invaded that country. On top of that, they have either killed or jailed anyone who opposed their policies.
I've even heard people complain the Dalai Lama ran Tibet like a medieval kingdom, enjoying himself while the people live in poverty, as if that is reason enough for China to invade them. However the Tibetans governed themselves is no issue to outsiders, the issue here is some country came in and invaded them for the obvious reasons of exploiting their resources. Did China tried to justify its actions then? With what slogans, "Free Tibet From Oppression"?