As actors age or contract out, I'd be fine with their characters dying, in most cases, rather than being recast or (shudder) rebooted. One possible upside of the MCU being a massive tapestry, instead of a collection of mostly unrelated standalone movies, is that rebooting just one character might be too strange, and even recasting a role that's appearing alongside a dozen familiar faces in the Avengers 6: Electric Boogaloo, could be seen as jarring. (Hey, it's Evans and Hemsworth and Johannson and WTH is Charlie Sheen doing pretending to be Robert Downey Jr?)
Robert Downey Jr. has done some fine work as Iron Man, but I think Age of Ultron would have been a fine end to his story, or, if nothing else, Civil War. Captain America already has two potential replacements, between Winter Soldier and Falcon, both of whom have picked up the shield in the comics, and War Machine (or even Rescue-Pepper!) could take over for Iron Man, when that time comes, rather than hire some schmuck to take over after Robert Downey Jr. retires to a Hugh-Hefner-esque lifestyle of slippers, bathrobes and supermodels.
Hawkeye hasn't really even *had* a story yet, and yet I'm tired of him, and would have much rather he bought it than Quicksilver, whose story was just beginning.
About the only one I'd be comfortable with being recast as they aged/contracted out would be Hemsworth as Thor, since he's playing an immortal, and he's, IMO, the weakest link as an actor. Even then, he's also got potential replacements, with his supporting cast, who may lack his power level, but that's hardly an issue on a team with Iron Man, the Vision and the Hulk. Sif's hardly a slouch (and Balder the Brave might be about to debut as well), and would occupy a different tier, but the team finds roles for Falcon and Black Widow, so she should be fine, being 'merely' ridiculously strong and capable.