Re: What sustains a bands popularity
Obvious, but write ridiculously good music. Also helps if the music strikes the right balance between being accessible, but not so much so that it gets old and stale after a few listens. Like your Tool example, it just gets better and better the more I listen to it.
I think it also helps to have an air of mystery and not throw yourself out there into the spotlight as much as possible so that every gets tired of seeing you. If people want to see you they should have to go to your show and see you through your music, not just open a tabloid and read everything about you. It also leads to more rabid fans because myths start to be form to explain everything that you don't give away for free, which leads to a cult following.
Don't sell out, in the literal sense, allowing your music to be used in movie sound tracks, and especially commercials/advertisements just leads to over saturation and people getting tired of you.