So let's say the Marvel Cinematic Universe is successful for a long period of time.
2030. Do we follow the real world timeline and Tony Stark is old now? Maybe RDJ pops up for cameos or something, but he's not flying around as Iron Man anymore.
Or do we adopt a sliding timeline like the comics and recast Tony Stark so he's eternally young? Because that would be weird too. I mean it's already weird for comics (come on, it's already stretching a bit to buy that Magneto was alive during World War 2, at some point that shit's just not gonna fly anymore), but it would be especially weird for movies.
So either we wind up with a cast of D-listers in Avengers 6, we screw up the timeline and continuity for the sake of keeping the popular heroes around, or we reboot. But then, a reboot presents its own set of problems. With one hero like Batman, it's easy, you just show the origin story again. But with a whole multi-franchise movie universe, how do you explain to the average audience that it's a reboot?
Basically what I'm getting at is this whole thing has to fall apart at some point, and that kinda sucks. It probably won't get a big grand finale, but it'll sputter and die out slowly. And we'll look back at the post-credits stinger to Avengers 3 and think, "Man, Avengers 4 could've been so cool, it's a shame the whole thing became such a mess."