[DennisP]

I actually have a lot of sympathy with this point of view. But there's a counterargument: if one fork has much greater hash power than another, then the minority is vulnerable to 51% attack from a fraction of the majority. So it's not like we can just ignore mining power, unless we make a fork that changes the hash function.

For Bitcoin, the difficulty adjustment would also need to be sped up, to keep the minority chain from being very slow for a while.

[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.