"Hey hey, my my/Rock'n'roll will never die"
-- said Neil Young
Well, I guess so far he's been right. Rock'n'roll hasn't died. But since Rust Never Sleeps came out in 1979 countless genres of music have disappeared or been relegated to dozy nothingness. Insignificantly small ones that blipped by on the radar. Big ones that defined an age. Some are even dying now.
But not rock'n'roll, that's survived well since the seventies. And whatever Nas has to say about it, not hip-hop either. Dancehall is doing remarkably well, again. Soca has faired better but shows no sign of seizing up. Punk is doing okay, if on a smaller, subcultural scale. Bollywood has produced some great material recently. Soukous too. And R&B -- well, you know R&B. How come this is so? Why do some genres seemingly go on forever, thirty years or more, while others fizzle out immediately?
Well, there's the obvious stuff. Vagueness is helpful. Very large size with different groups forming different understandings is probably a good idea. Chance definitely has something to with it, with the large arbitrariness of how labels are applied denying new ones survival or creating ones that are especially strong.
In general, for almost any genre that manages to stay strong over nearly thirty years, the ability to absorb generation shifts is incredibly important. Something like "rock" solves this by having each generation take over the label but kick out its current practitioners - there was no opposition in both prog rock, pub rock and punk rock being genres of "rock" in Britain. Thus the inevitable generation putsches don't necessarily involve getting rid of genres, and different groups and scenes in different circumstances all adopt the label.
Soca, dancehall and Bollywood (to name three) are different though - here it's pretty much one continuous scene with stationary and/or gradually shifting centers, yet they've remained vital throughout. I think the other possibility for a scene that will outlast its original generation is music that's very clearly functional, with a group of actors (sound systems, carnival organisers, movie makers) competing against each other and demanding good quality product from musicians. This way institutions benefit from constant development and encourage it, gradually brining in young talent instead of excluding it.
This is all idle speculation in any case. I'd love to hear anyone with a really long-lasting genre that doesn't fit either the "functional music" or "vague term that stands for something rather than represents a scene" criteria. In particular, I think the Jazz scene might have gone beyond either (with non-functional music and respect for the elders and institutions), and I do think some genres will change around over time.