I would be quite worried about advances in quantum computers if I had any Bitcoin after watching this DEFCON talk:
https://www.youtube.com/watch?v=OkVYJx1iLNsEvery time I mention quantum computing as a threat to crypto (which I have been for years), I get downvoted to oblivion. I guess we have a lot of HODLers here. A bet on crypto is a bet against quantum computing.
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?
Why though? Who is to decide which point of the blockchain is the good one and which blocks to “reject”?
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.