What's interesting with the MCU, as opposed to Superman, Sherlock, James Bond, and the previous Spiderman movies, is that the characters have aged. Twenty-three movies have taken place in some kind of chronological time frame. Those other franchises just reboot after a few movies and bring in a new actor to play the role without any real connection to the previous story lines. The same often happens in the comics as well. The other thing that happens in the comics is that characters don't get older. It's been a long time since I read any of the Marvel comics, but are Reed and Sue Richards still the same age? Is Peter Parker still in high school? It's going to be hard to see anyone else play Tony Stark or Steve Rogers. They've just passed the Captain America mantle to Sam Wilson, but that's not the same as continuing the same character with a new actor. I can't imagine anyone but Chris Hemsworth playing Thor. There have been three actors playing Bruce Banner/Hulk, but that franchise is dead, and the Hulk has become a secondary character. Kevin Feige and his team have already moved on to a number of other Marvel characters, so maybe Stark and Rogers truly are done. Time will tell, I suppose.