Copyright law would not prevent us from creating derivatives of Steamboat Willie, but trademark law probably would. We could still sell Steamboat Willie on those dirt-cheap DVDs, though, as-is. For what that's worth... it's really of primarily historical interest, I doubt modern audiences will find that much to be terribly entertained by in it.

I don't have a problem with this. While it's not likely to any of the tastes of anybody reading this sentence, since Mickey Mouse right now is mostly being used to entertain those too young to read, he's still actively producing things. (And while Mickey Mouse Clubhouse is hardly a masterpiece for adults.... there's much worse out there to have to listen to ad infinitum.) The offensive thing about infinite copyright extension is locking up things that the creators are done with, and have been for decades, but won't let anyone else do anything either, thus essentially killing the cultural artifact in question.