I dunno. Marvel will lose Downey as he's getting up there in age (but Jackman did Wolverine for 20 years, so I guess we could see Downey for a bit longer, especially as he doesn't do most of his own stunts). The Chris' will stay. ScarJo is a toss-up (but probably would stay?), Renner would probably stay. As would Chedale.
I dunno about Ruffalo, but I could maybe see him sticking around.
In fact, in an ideal world, Downey as Stark would be like Sam L. Jackson's Fury, behind the scenes and doing like 1-2 scenes per movie. This would allow them to keep the older cast/characters while bouncing off their Phase 5(?)+ new characters like Starlord, Captain Marvel/Carol Danvers, etc.