The recasting of characters like Rhodey, Fandral and Bruce Banner still smarts, with the latter even causing The Incredible Hulk to feel like it doesn't really belong with the rest of Marvel's oeuvre. To recast tentpole characters like Tony Stark or Thor would be too blatantly obvious and feel like a disconnect from everything that has come before. More than that, it just frankly wouldn't be the same; at this point, the line is so thin between Robert Downey Jr and Tony Stark that you'd be forgiven for getting them mixed up on a daily basis. To imagine anyone else in the role is folly. Thor... maybe less so, but Hemsworth has been enough of a fixture so far that it'd be far too noticeable.