I've seen many cases of community decline over the years (message boards, IRC channels, EVE online, Super Happy Dev House, Squaw Valley, etc. etc.). It seems that anything truly great inevitably enters an an eternal september
The greatness attracts crowds, crowds are average by definition unless you only attract some subset of people that will make things awesome. But the best events have some amount of self-selection. The playa's harsh environment appears to filter out a good amount of randos because it's hard, hot, and expensive. The quality of the burn still obvious attracts many people.
Some open questions:
* Has burning man reached a growth point where its exhausting the people who might want to come? I.e. out of everyone in the world, has everyone who might want to come mostly already come?
* Does the harshness of the playa act more as an effective self-selector or hazing?
* Why did I drink so much coffee today?