Punk for me felt officially dead when I noticed there were songs accepted as proper punk that were about trivial things like hitting on girls and driving cool cars in the US. There is no one point in time when this happened - for example the Ramones sang about all kinds of largely irrelevant nonsense and I could accept that. It relates more to when I personally discovered this
and felt the rebel spirit in punk was almost completely gone from the music of the contemporary punk mainstream.
Today, I wouldn't say movements die though. Only in a highly relative and subjective way
I think the idea that a movement dies when it becomes mainstream is pretty off though. It wasn't popularity which killed off prog in the 70s .. It probably had more to do with the subsequent rise in popularity of guys like this and their beliefs :
Any one of us can of course appreciate Sex Pistols and Pink Floyd at the same time, but the truly massive scale popular opinion can't. Different bands and genres are competing for a limited resource, our collective interest, and thus there are winners and losers. I think a movement would die when (or rather "if") there's no interest in it anymore or, more arguably, when it's changed so much that it's become something completely different and noone has interest in the original shape of it. In the latter example, it may have passed on some genes to it's offspring.