Yeah, I wonder what the earliest article about that is. I did a brief search for the decline of Burning Man found a Wired article from 1997[1] (and of course numerous people talking about the decline later, like this blog post from 2004[2]).
It reminds me of all those "I'm 13, is it too late for me to start learning programming/body building/learning the guitar/studying Japanese/etc." type posts. If you don't do something you're interested in because you think its too late, there's a good chance that a decade later you'll be kicking yourself for not having done it.
[1] https://www.wired.com/1997/07/burning-man-burnout/ [2] https://eplaya.burningman.org/viewtopic.php?t=7447