Surely this shit is an unsustainable highwire act, considering the finances involved? Not just the Marvel film universe, which was at least conceived as a thought-out, interconnected entertainment division, but more the other studios which are trying to string together whatever they have the rights to in to a comparable brand. The DC films will struggle because they are now convinced that everything needs to be grey and gritty to match the Batman films, yet they don't seem to have the balls to go for something bold like Kingdom Come (which would be pretty apocalyptic and wouldn't leave much room for ongoing sequelisation). Sony's Spider-Man franchise is also pretty doomed, considering the narrow roster of characters they're trying to mine for material.
Comics have obviously stood the test of time, so it's not like there's a lack of popularity for them as a medium, but it's usually gone through boom and bust cycles every few years. Superman in '78, Batman in '89, all led to pretty quickly to a fallow few years where people got bored and wandered off. This one has been pretty constant since Spider-Man in '02, although it's had its share of X-Men 3/Spider-Man 3/Fantastic Four/Origins-Wolverine (critical if not commercial) failures, but surely the casual moviegoer is going to get fatigued at having to keep on top of all this baffling story guff sooner or later? How long can a $200 million franchise film rely on teenagers giving a shit about a plot point from five films previous released under a totally different title?