I don't think there is any consensus protocol that can survive an attack by 90% of nodes.

That is true. However, to the point made above, proof of work is actually using a scarce resource. If you fork a proof of work chain, the amount of mining power remains the same. You can’t mine both chains with the same resources. If you fork a proof of stake chain… everything just copies over. And then maybe if people place nicely in the name of the being good crypto citizens they agree to only focus on one chain.

The scenario sounds extremely implausible; a chain with 90% colluding actors would be worthless, and the remaining 10% of honest users would likely exit to another chain that is not majority run by malicious nodes.

For example: migrating to a new chain ID and restarting with a validator set limited only to the public keys of the 10% who are acting honestly, and loosening that restriction slowly over time.

The attack would end up being extremely costly: not only because it makes the original chain’s token price worthless (who wants to be on a malicious chain?), but because the attacker may also have their deposit slashed in the new chain by the now-majority of honest users defending it.

The 90% is an arbitrary and unnecessary figure. Note it is not 90% of actors either. It is the bag holders of the bulk of the wealth. Far easier to imagine. And if you try to fork off into a world where the most important financial players have nothing, you will not be taken seriously. The money is more important than the crypto.