Avengers has the cast signed on for 3 Avengers films, 3 solo films, and at least 1 or 2 or 3 cameos. So I am guessing that, like Downey Jr., Evans, Hemsworth, Johansson, Renner, they will all leave as soon as their contracts are up. Sure some might stick around, but the majority will leave. And there are so many Avengers that they wouldn't need to recast, and so many Avengers and so many villains and so many stories, that I would HATE if they ended the films at Avengers 3. So my idea has always been that they should just bring in more Avengers when the others leave. But stagger the rotation to that its not like all the originals are gone, now lets bring in the new guys. Its more like Law and Order where the new guys come in while the originals are still there.
So Avengers 1:
Captain America, Iron Man, Thor, Hulk, Hawkeye, Black Widow, Nick Fury, Maria Hill
Captain America, Iron Man, Thor, Hulk, Hawkeye, Black Widow, maybe Fury, maybe Hill, and Ant Man and Wasp
Versus (lets say): Thanos
Captain America, Iron Man, Thor, Hulk, Hawkeye, Black Widow, maybe Fury, maybe Hill, and Ant Man and Wasp, and Falcon
Versus (lets say): Ultron
Ant Man and Wasp, Falcon, Carol Danvers/Captain Marvel, She-Hulk, Mocking Bird, Vision, maybe Wonder Man
Versus (Lets say): Kang the Conquerer
Falcon, Carol Danvers/Captain Marvel, She-Hulk, Mocking Bird, Vision, Wonder Man, Black Panther, Bucky,
Versus (maybe): Namor
Carol Danvers/Captain Marvel, She-Hulk, Mocking Bird, Vision, Wonder Man, Black Panther, Bucky, Namor
Versus (I don't know): Grimm Reaper and an army of the dead
Vision, Black Panther, Bucky, Namor, Scott Lang/Ant Man, Spider-Woman, Valkyrie, Luke Cage
Versus (maybe): Korvac (spelling?)
Vision, Namor, Scott Lang/Ant Man, Spider-Woman, Valkyrie, Luke Cage, Iron Fist, Beast
Versus (how about): Baron Zemo and the Masters of Evil
In my opinion the films could go on and on and on without recasting anyone. This is just my very simple example. Who is to say that one guy, Renner maybe, wont want to stay and be Hawkeye for another 2 Avenger films. Or maybe Hemsworth will want to return around Avengers 6. And it wouldn't be like these guys would just pop up out of nowhere. I see many starting out in solo films like the originals did and like Ant Man will be doing soon. So Danvers/Marvel, or Black Panther will rise from solo films. Falcon and Bucky will first be seen in the Captain America films. And characters like Vision or Namor will come from the Avengers films themselves. And others can be introduced without much intro, like She-Hulk or Valkyrie. And of coarse the choices of who would be an Avenger and when would be based on multiple factors, who has a solo film, or who the enemy they are facing is (like maybe the Avengers face the Masters of Evil and gain Wonder Man as a member in the end or maybe they fight Ultron and gain the Vision as an Avenger), and who fits best into the Marvel MU at the moment. For example, maybe the team begins to be more and more former villains, or maybe the MU Earth becomes more aware of aliens and the team has more cosmic heroes. Or maybe they want the team to be more international, so they have Bucky Cap (America), Black Panther (Africa), Captain Britain (England), Wolverine (Canada), and others, on the same team.
AND they can and probably will eventually recast characters like Captain America and Stark.
Still, and to make a LONG story shorter, I think if they can stagger the roster rotation, they can slowly change the cast and keep the films going without long into the multiple digits.
My roster change idea also includes bigger ideas, like Marvel movie studios doing 15 minutes of previews ahead of most feature films to introduce some characters they sort of want to experiment with and test on the public. So maybe thats where we could first see Luke Cage and Iron Fist.
Now I will stop before I start to ramble more than I am.