Quantum is an example of a slow-moving disaster risk. It’s not going to pop out of nowhere. Incremental progress will be observable long before it’s a critical issue.

Bitcoin can fork in additional signature schemes and/or additional proof-of-work constraints. Even if a hard fork is required for something, Bitcoin still wins.

Consider a hypothetical death-knell bug that requires a hard fork to fix. One or more competing forks will emerge. One of those will achieve economic consensus (has all the value). We’ll call that chain “Bitcoin” and keep moving. Anyone who argues “that’s not real Bitcoin” is free to remain on the dead chain.

I don’t think such a situation like this is likely to come to pass. But if it did, everything would be OK, eventually.