Let us assume a miner decides to roll back segregated witness on what is presently the most valuable chain.
There is a non-zero chance of this happening- one which Core-ists seem never to discuss as a looming concern or even a possibility. In fact, it is generally waived away as BCash FUDsterism.
It's been pointed out time and time again that 80% of reachable nodes are enforcing Segwit. By any measurable statistics we have, such a fork would be ignored by most of the network. Miners would be on their own hard fork, although really old nodes from 3+ years ago who can barely keep up with the network anyway would be compatible. It's extremely unlikely most of the network will downgrade their software to accept such a hard fork. Why would they do that? Because they want dishonest miners to steal everyone's money?
*Reversing the attack would negate immutability
Quite the opposite. For a strong majority of the network, hard forking our software to remove Segwit would negate immutability.