I am saying that I doubt Chris Evans, Robert Downey Jr, Chris Hemsworth, Scarlett Johansson, Sam Jackson, and the rest, will stick around after their contracts are up. They probably have the exact amount of movies and types they are going to be in, I hear Evans is signed on to 9 movies (3 Captain Americas, 3 Avengers, and 3 cameos maybe). But it would be a shame to end the franchise there because the original cast wants to leave, and instead of recasting just bring in new characters played by new actors. And to keep the flow stagger the roster rotation like Law and Order does. So not every character is leaving and ever new character is entering at the same times. I saw bring in Hank Pym and Janet in Avengers 2 so that the audience will get to know them so that by Avengers 4 (the first Avengers after the original cast has left) you will still have Hank and Janet there. Then when Hank and Janet leave around movie 5 or 6 you can bring in Avenger characters to replace them too. Then when the cast from Avengers 4 leaves around Avengers 7 you can replace them too with new characters. There would always be a few characters that would stick around since their contracts started later.
I am making it a given that the actors would probably be signed to at least 3 Avengers films, and would probably leave once those 3 films were done.
These are just random casting to help over explain my idea:
Avengers 1: Cap, Iron Man, Thor, Black Widow, Hawkeye, Hulk, Fury, Coulsen, Hill
Avengers 2: Cap, Iron Man, Thor, Black Widow, Hawkeye, Hulk, Fury, Coulsen, Hill, and Hank Pym and Wasp
Avengers 3: Cap, Iron Man, Thor, Black Widow, Hawkeye, Hulk, Fury, Coulsen, Hill, and Hank Pym and Wasp
Avengers 4: Hank Pym and Wasp, She-Hulk, Vision, Scarlet Witch, Quicksilver, Wonder Man, and SHIELD agent Gabe Jones
Avengers 5: She-Hulk, Vision, Scarlet Witch, Quicksilver, Wonder Man, SHIELD agent Gabe Jones, and Ms. Marvel, Black Panther
Avengers 6: She-Hulk, Vision, Scarlet Witch, Quicksilver, Wonder Man, SHIELD agent Gabe Jones, and Ms. Marvel, Black Panther
Avengers 7: Vision, Ms. Marvel, Black Panther, Valkyrie, Firestar, Falcon, Hercules, Agent Clayton
Avengers 8: Vision, Firestar, Falcon, Hercules, Agent Clayton, Mockingbird, Luke Cage, Iron Fist
Avengers 9: Firestar, Falcon, Hercules, Agent Clayton, Mockingbird, Luke Cage, Iron Fist, Photon
Avengers 10: Mockingbird, Luke Cage, Iron Fist, Photon, Spider-Woman, Jessica Drew, Black Goliath, agent Bridge
Etc.
(Vision I could see being a CG robot like iRobot and animated with motion capture things, the actor could easily be replaced)
The movies could keep going and going and going. These were just random examples. The plots could be whatever...
Eventually Rick Jones/Captain Marvel, Bucky Barnes/Captain America, Thunderstrike, and others could make their way through.