If quantum allows for dozens of orders of magnitude faster cryptographic computations (no serious quantum people are saying this, they are saying the exact opposite — this is not how quantum computing works, it isn’t magic) everyone can just use bigger key sizes and all miners will adopt the new quantum strategies. We will end up with every wallet being a n-key multisig underneath and the block difficulty will rise exponentially to reflect the new ease of mining. But within a very short time, such breakthroughs would be priced into the computation difficulty of Bitcoin.