Marvel, sure. But after Infinity War, it will be less popular. What are they going to do?
They will have saved the universe; no matter what they do after, it will either be underwhelming or the same old things. Also, by the time Infinity War 2 hits in 2019, RDJ will have been Tony for 11 years. You really think he'll want to go on for another 6 in non-cameos?
Actors aren't like drawings, you can't keep them around forever. RDJ will never have any problem working, ScarJo and Ruffalo already have a good career outside of the MCU, Renner gives no shit, etc.
The point is, they will have to change their cast at some point. The shared universe makes it that they can't just replace one actor, it would feel ridiculous if Evans and NuTony acted as if they had a past. They can't reboot and hope to have the same success either, people are way too invested in the first MCU to jumb back in a second one and loving it as much so soon. (No, it's not like having 3 Batman actors in 8 years)
That only leaves you with introducing new characters. Either more B and C lister that no one cares about, or you do things like, I dunno. A female Thor? A black Cap? A black female Iron Man?
What's happening in the comics is not just for diversity's sake. There are creating a precedent for the movies. It will be much easier to swallow RDJ being replaced by a black teenage girl if "it's just like the comics". But moviegoers will not buy that for long.
I'm far from my starting point by now, but still; the interest in the MCU is not eternal. Phase 3 will be highly succesful. We will get a Phase 4, but the decline in success will be clear.
And that's just Marvel. For WB and Sony, they will stop way before. Again, we will still have a few Batman and Xmen movies once in a while, but do you really think that they will want to be against the juggernaut that is the MCU for much longer? They will push for a new trend, like every studios without superheroes.