Assume GPL V 2.1 game out and it includes this clause. Not any of the other DRM related stuff that seems DOA in GPL V 3, just this new change.
I'd say a large portion of the people who released GPL V2 code would happily relicense to the 2.1. Any of the old code is STILL V2, but any new patches / releases will be 2.1. So the old code will die, while new releases will live.
It is essentially self-forking, but only code owners can use a new license.