Twitter has some problems too. People do change their username, and every time they do, all previous tweets mentioning them by @username lead to dead ends.
But it may very well be for good reason. E.g. lets say an account is taken over by someone who changes the account into something suitably offensive after obtaining a lot of mentions. It would seem that treating a change as basically "this is a new account now" is the safest alternative in some respects.