Because difficulty does not adapt dynamically, if the miners do not move then the fork will be defunct because it will take literally days to weeks to mine a block, and to mine the 216 blocks that would trigger a difficulty adjustment would also take proportionately longer. So transactions would sit in the mempool and the currency would be mostly useless.

Fair point. Maybe you could hardcode a lower difficulty in the fork for the next couple dozen blocks, and schedule an adjustment after that?