Originally Posted by bermuda
A lot of good points, some do stray from the 'sustaining' aspect of the question though.
And that's the quandary: a band staying their course, while the public changes.
It's important to remember, the public decides how much popularity a band gets, and how long they can keep it. That doesn't mean that a band needs to constantly change its music or style to remain popular, but there's a fine line between sticking to one's roots, and being a chameleon. I don't know of anyone who's sustained a career strictly doing either.
A band that can be relevant and consistent (not to be confused with predictable) has a shot at continued success over the long term. There's obviously never a guarantee, and there are undoubtedly some exceptions, but some of the biggest bands/artists that have transcended a generation or two have done so by maintaining consistency, while staying relevant. The Stones, U2, Rush, Jimmy Buffett, and Neil Diamond come immediately to mind.
Heck, I'm in a band that's done darn well over 27 years of releasing albums and touring! It's easy to say that Weird Al's just a novelty and doesn't compare to mainstream artists, but don't underestimate his ability to fill 6,000 seat halls (and larger.) As I mentioned, there's consistency - always doing parody & satire - and, relevance - parodying music and artists that are current, or at least timeless, and which have mass appeal at any given time. No sell-out, just doing what he's always done, and keeping it fresh at the same. That's also why the demographic is so wide (the proverbial "six to sixty" age group) and what's kept Al going and growing: there are always new fans, yet the the old fans also stick around.
And there we are again, it's the fans - the public - who decides how long a band can successfully stick around.