... Miners ... can't break the rules of the system enforced by the nodes... if they do, they're not miners anymore as far as the nodes are concerned.
Yes, they are miners -- for the updated nodes/wallets/users.
The legacy nodes/wallets/users won't have any miners, and thus become subject to savage rapings by any kid with a couple of vidya cards.
Bonus points: no tx confirmations forever because outrageous difficulty/hashpower ratio, no diff. adjustment forever, because see aforementioned.
That's how hard forks work, always assumed it was understood.