There are plenty of methods for direct transfer already, I don't think a different IP (with its likely non-trivial setup) would make much of a difference for Dr

There are plenty of methods for direct transfer already, I don't think a different IP (with its likely non-trivial setup) would make much of a difference for Dropbox. Keep in mind that would you describe would require both computers to be on at the same time, and that often doesn't happen either. Dropbox would be here to stay even in an IPv6 world.