i think you might underestimate AC just a bit. granted, they're much more likely to settle into the sonic youth/flaming lips career path/popularity. but if they hit their 95% PECOTA projection they could get to ... well, not radiohead level, but something approaching that. in any case, 20 years from now there's no question they'll be far more relevant than kings of leon or the killers. but that won't really be a select group.
I might be underestimating - sure. But I also don't see Animal Collective's music or stage show translating in a space much bigger than Constitution Hall. Can you imagine seeing Animal Collective at the Verizon Center? Or even the Patriot Center for that matter? I'm better able to accept the idea that they could become a group playing sheds, but even then - that's a lot of people for a band whose music is designed to be avant garde.
I also think it is debatable how relevant they'll be 20 years from now. Relevance is an interesting question in general, as it depends on how you define it.
sure, i can imagine seeing animal collective at verizon center. i'm imagining it right now. there it is. i see it. damn, it's fucking empty in there! who booked this show?!?!
seriously though, say there are 1,500 people at an AC show right now and say there are 8,000 people at a KOL show. i bet if you asked everyone at their respective show if the band they were seeing was one of their very favorite bands you'd end up with roughly the same number of "yes" responses. i don't see AC losing any fans in the near future. i think they'll only gain fans, even if they don't make a record as good as "MPP" again. KOL/killers don't have that loyalty, or the overwhelming popularity of pearl jam back in the day. like, i know KOL pretty much sold out the patriot center, but back in like 94 PJ sold out the patriot center in a matter of minutes, and that was before internet shenanigans.