I don't think recasting Stark is quite so easy a task as Rhodey or Banner. I suspect that when RDJ leaves, so goes Stark (though not necessarily Iron Man). The same should be true of most of the cast of the MCU. Their names and titles might continue without them to extend the franchises though: Captain America is a title that can be carried by any number of people, Thor is a name that can be claimed by anyone worthy, etc. I'd much rather see characters leave with consequences to their and others' actions than be replaced with someone "close enough" and if the MCU continues unbroken I see no reason to plan for recasting anyone. Sign the talent to however many appearances it will take to tell their story and when the story is done being told, so are they.