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.
