I can tell you from having done on-line flying since the start of it (Air Warrior in 1988) that very few people stay forever. It's not anything that the game is doing or not doing, it is that people eventually drift off, enter different stages in their lives, get different hobbies, etc.
I've written this before, but all of these discussions -- nearly verbatim -- were things people said in 1993, 1995, 2000, and so on. "Things aren't like they were in the good ol' days!" But really, they are just as good as the good ol' days, and the style of play and the style of the community is not significantly different over the past 2 decades.
If things seem mundane to you, it's because you have done all of the things that were brand new to you when you first started. You reached your plateau, leveled out, and didn't try for more. Maybe for you, there is no more -- you're the best pilot you can ever be, have become as good as you will be in every plane and vehicle, have become expert in every special event. My guess, though, is that most players complaining of burn out peaked out in their own particular ruts and haven't, for example, gotten a lot more involved in special events (which are immediately much different than the Main Arena and a whole new realm to explore).
Some of you guys are like the high-school kids who sit around during summer vacation and say, "I'm bored. There's nothing to do." What they mean is that they've gotten their fill of the mall -- but they haven't started reading any great books, playing basketball with their local pals, watching a collection of the greatest movies ever made, learning about computer programming, building some models, getting into gaming with their pals, learning about the stock market, or whatever (i.e., moving beyond their rut of just going to the mall).
This game has a lot of possibilities and can be what you make of it.