At this point, I think "burnout" varies case by case depending on the studio. In Marvel's case, I don't see it happening at least till Infinity War.
Outside of them leading the market at the moment with no sign of that changing in the near future, the GA seems to be aware of their plans based on my personal experience. I've heard plenty of casual moviegoers in the past 2 weeks bring up Thanos, how he'll be in Avengers 3 and how everything's been building up to that. So if they can communicate that message across to their overall audience and get them on board with it, I imagine the audience would stick to Marvel till the finale. Unless they drop the ball in terms of quality, which goes without saying.
On top of that, a significant factor in burning out your audience would be the lack of diversity. What killed westerners, based on what I've heard, is that they all eventually turned into the same thing. That doesn't seem to be the case with the Phase 3 lineup, or even with the overall lineup we had this year from all studios.
Furthermore, superheroes are prone to a lot more variety than other genres. Guardians and Winter Soldier were polar opposites in terms of almost everything, the big exception being the idea of having superhero protagonists. And even then, the idea of the "hero" is almost universal, is it not? It's by no means exclusive to just superheroes. Then there's also the idea of the "antihero", which I would argue fits the Guardians better, as well as the upcoming Suicide Squad. Point is, even the black-and-white definition of "hero" within superhero culture is starting to become malleable with these films.
What I think could happen, at least for the foreseeable future, is a "survival of the fittest" scenario where the best films get praised and the ones who can't keep up get left behind. We've already seen a glimpse of that this year with TASM2, the only mixed-to-negative received film (not counting Turtles) underperforming while the rest overperformed. I've seen some compare 2010 Marvel to 2000's Pixar, and I couldn't agree more. There were plenty of CG animation films last decade (and still are), but Pixar managed to stay on top of the market through the consistent quality they put out.
On the other hand, Sony seems to be in panic mode and not have any clue to what they're doing. Even if the Sinister Six film concept delivers, I don't see how the same team that can barely market Spider-Man be able to market such an idea. The female spinoff may have come off as a progressive novelty a few weeks back, but it could easily get buried by Wonder Woman and Captain Marvel at this point, coming around the same time. I see TASM3 being Sony's last Marvel film, assuming they even manage to get there successfully.
WB is a complete wild card at this point, but if the rest of their lineup is on par with MOS quality-wise, they'll also be in trouble.
Fox is the best one off after Marvel, at least in terms of X-Men. I see the X-Men franchise sticking around for a while, especially if they take a successful young Scott/Jean/Storm off the ground. In terms of the FF, "unsuccessful" is an understatement. I wouldn't even count the FF films as an upcoming CBM's, since chances are the first one will be too low-key for the GA to notice, which is what Fox is clearly aiming for. Same thing with the second one, assuming the revenue from the first justifies making it in the first place. But of course.
Finally, I'd like to conclude with a personal note. I'm enjoying all the superhero TV shows out this fall - Arrow, Flash, SHIELD, Gotham - but my schedule is too packed for me to keep up with all of them. As a result, I dropped Gotham because it was the least interesting for me. I will be picking it up again when I get the chance, but it's off my mind for now. I'd also like to give Constantine a try at one point, but I can't at the moment without dropping one of the other 3. Now of course, take into account that if I wasn't a hardcore fan of these things, I'd probably drop them for good and never pick them up again. That and there's no telling when I will have the time for them. Essentially, superhero survival of the fittest has already begun, at least for me.
