One thing that is not addressed: say this quantum attack happens tomorrow and everyone agrees it was an attack, what would prevent the community (miners, node operators, and users) to hard fork the chain at a snapshot before the attack, patch the protocol, and call that Bitcoin? There would be loss of value of course, but it is not unrecoverable.

It’s worth remembering that Ethereum forked for much less (not even a bug in the protocol, but a bug in a private application running on the protocol) and nobody seems too upset about it a decade later.