Unless advances in QC could rewrite the blockchain then there's not much to worry about. If the crypto algorithms are compromised, you coins are pretty much frozen on the chain until a new algorithms are implemented. Are you're arguing QC makes signatures/verification/mining impossible?
Before things were frozen you would have the biggest crash in market history. Then it would be like a futures market that has gone limit down. The panic during this time would be unbelievable.
Then when it reopens it would crash further and effectively end trust in crypto.
Think twice. Everyone who hosts the blockchain would decide to stop because he invested in crypto, at least with some hardware costs. Beside of the small group of people that owns a quantum computer. I don't expect that this group is >50% of the people that hosts the blockchain.
You don't need >50% of bad actors to compromise the blockchain, but rather >50% of the total hashing power. This could very well be achievable by a small group of people with QC at some point.
