Anything can happen in an Alternate Universe, since it isn't our world. People could be talking lizards over there, or maybe their Abe Lincoln was never assassinated, or maybe they can all fly. But if heroes from our world visit the Alternate Universe, they won't care what happens there. Typically, neither do the audience. Things are even worse if there's only one world, but Time Travel can change it back and forth. People in the future can die a hundred times each, but since the Time Travel will bring them back, who cares? Compare For Want of a Nail, which is the Time Travel equivalent.