Second, it doesn't change the fact that you'd still have this insanely powerful quantum supercomputer running for millions of years before you have a chance at double-spending a single coin. Not economically viable as I said before.

Various people do consider ECDSA to be effectively broken with quantum computers. It's hard to estimate what a quantum computer of a certain power is going to cost in 20 years.

That said, I don't need to capture enough coins to pay for the attack. I can buy options on failing bitcoin price and attack. If it's known that there's an attacker who randomly hijacks transactions the bitcoin price takes a blow.