I think you completely missed the threat of QCs on blockchains. Nothing to do with mining, everything to do with data security.

A QC can quickly reverse a private key from a public key. The ability to do with brings the value of BTC to zero. If I am the chinese goverment with the first 512 qbit machine (some say you need 4096 qubits) and I attack satoshi's wallet and withdraw funds from it, it will demonstate, loudly, that BTC is worthless.

Luckily, this is probably a few years off.