People often bring up the Bitcoin algorithm to make arguments against it, but don't seem to acknowledge the fact that the protocol is mutable.

If the sha-256 algorithm was cracked such that BTC blocks could be solved instantly, the existing miners would have to choose between:

1. No more income, or

2. Adopt a quantum-resistant protocol.

Market economics being what they are, I think it's safe to assume that BTC would survive the "quantum apocalypse." There's too much money at stake for any other choice to be the logical outcome.