There is no ruler. The user can choose the software to run. It's possible there will be multiple protocols that claim to be Bitcoin. It would be nice if some general consensus emerged not to use the Bitcoin name for specifications that introduce substantial changes to the underlying economics or reward structure of the protocol. But even if there's no consensus on semantics, each group can choose to ignore the other and the only consequence will be that I have to check who is speaking before I conclude they mean the same thing I do when I say "Bitcoin".