There were people already saying "it's not cool any more" even in the 90s. That was in fact how I first heard about Burning Man, due to someone complaining about it not being cool anymore, around 97 or so. Very likely people have pretty much always said it.
It's like the old joke about how the first car race happened the day someone built the second car:
Q: When did someone first say Burning Man wasn't cool any more?
A: The day after the second Burning Man.