Seems completely logical.
I always figured that more established actors, like Downy, Evans, Garrofolo (sp?), Hemsworth and Johannsen would lose interest in the roles they have been playing for a long time. In addition, those stars have more and more other options and their salary increases. It makes sense to replace them with younger, less well-known actors, or to kill off their characters.
Unless I am mistaken, there is comic book precedent for Bucky Barnes to assume the mantle of Captain America.