Just to clarify, IF they ever were meant to be brought back forever, it was very, very early in the program, meaning many years ago. I say if, because so far, no one has come up with viable proof that they ever were designed that way to begin with. No documentation, no agreement forms, no promotional smoking guns. Even if they were it was for just the resort that one was staying at when they purchased them.
Just as soon as one of the neophyte imagineers or food planning people were able to eradicate their heads from their butt cheeks, that little offer, if it ever existed, was killed off. Since that time, any purchased before that moment of Disney enlightenment, were honored but any mugs sold after were sold as length of stay only. A campaign of ignorance happened after that with people justifying the use of them on subsequent visits as being what Disney intended. No matter what name one puts on it, stealing is stealing...they have to live with it and sadly I'm sure it's not bothering any of them at all.
So, as far as I am concern, the fact that Disney went to the trouble and expense to change the system to prevent pilferage, that it should be pretty good proof that it they never intend to have people just take things on a continuous basis.