The Marvel Cinematic Universe has built its success on taking a structure that has been successful in the comic book world for decades, that of the interconnected universe, and translating it to the big screen. However, the franchise is about to be forced to deal with a major change, and it's something that comic books can't help with. The MCU is going to need to replace some heroes, and they're never coming back.
The "replacement hero" is a fairly common move that numerous superhero comics have used over the years. If things start to get a little stale or slow, the well-known man behind the superhero mask gets killed off or severely injured to the point that they can no longer play the part. However, since the comic book is well known and popular, it continues on, with a new character playing the part of the hero.
As we sit on the verge of the release of Avengers: Infinity War at the end of this week, and with Phase Three of the Marvel Cinematic Universe coming to a close next year with Avengers 4, Marvel is faced with something similar as several contracts are getting ready to expire. We fully expect to see characters like Steve Rogers and Tony Stark written out of the franchise one way or another. They might get killed, they might retire, but somehow, they will cease to be significant parts of these films.
However, like the comics, these characters are incredibly popular and it's difficult to imagine the MCU without an Iron Man or a Captain America. This has led many to suggest that other heroes could take on these mantles when the likes of Chris Evans and Robert Downey Jr. say goodbye. Perhaps Sam Wilson will get promoted from being Falcon to becoming the new Captain America, something which has happened in the comics. Some, like myself, have suggested that Letitia Wright's tech genius Shuri could be the new Iron Man. Either or both of these things could very well happen, however, there is a fundamental difference between when this happens in the comics and when it happens (if it does) in the movies.
In the movies, the original heroes are never coming back.
You see, there's an unspoken contract between the comics and the reader every time a hero gets replaced. They'll be back eventually. The replacement hero is always a short break before something happens that allows the original hero to return. The death was faked. Time travel resets things. Magic heals wounds. One way or another, the hero you knew and loved will be back to fight again. The replacement hero only usually works because the original hero is going to return. Part of the point is to remind readers why they loved the original hero in the first place. That won't be what happens in the movies. Marvel is wading into uncharted territory when it comes to the idea of what to do when this current crop of heroes takes their leave. Whatever choice, or choices, are made are going to be entirely new for Marvel, with no guarantee of success.
Even if nobody is actually killed off in the next two Avengers movies, the actors seem ready to say goodbye to these roles they've been playing for a decade. Even if they were willing to return, eventually they would just become too old to helm a major action franchise. Comic book characters never age. Real people do.
Marvel has a few different options on how to handle this. They could recast the parts and have them continue on as if nothing changed, simply having the heroes played by other actors, but it's been said that at least in the case of Robert Downey Jr. that won't happen. It seems unlikely to work well for most of the major Avengers, these actors have become synonymous with the roles. Alternatively, they could simply remove the characters from the MCU entirely. They can move forward focusing on other characters and see the world that has been created move forward without a Captain America. The third option would be to see the heroes taken over by new people, such as the aforementioned Sam Wilson becoming the new Captain America, but if that happens, then he is, and will be, the new Cap, until such time as Anthony Mackie himself is ready to leave the MCU.
If any of the actors leaving the MCU see their characters survive, there's always the possibility for a brief return in the future. Tony Stark could come out of superhero retirement for one important mission in five years. Even if characters do die, the various ways heroes have returned in the comics could be used to bring them back on film, but it would only be for a special event, not the long term.
In the end, when the Marvel Cinematic Universe makes whatever major changes it has in store, those changes are for the long term. The MCU made history by creating the concept of the cinematic universe. It's about to do so again, but going to places that even the comics have never been: Real, permanent, change.