[psyc]

This is all true. The way I think about it is this would be just like any other 51% attack, except that the motivation is specifically to compete by killing the former network. But game theoretically, it's still an attack, not a valid way to achieve consensus. So, as you say, the correct response is for the original network to hard fork.

To be clear, I'm not saying 2X is itself an attack (IMO, awfully close to crossing the line, though). I'm assuming that in your scenario, some mining faction is spending part of its compute on running 2X, and part on explicitly 51% attacking the original network.