Without quantum computing a 51% attack will never work.
After quantum computing becomes mainstream, then even a 51% attack by means of quantum computing will be unrealistic...
I don't think this type of attack is going to happen... Ever.
Bitcoin will probably be replaced by something else by the time "quantum computing goes mainstream".
I don't think BitCoin can be replaced in the common sense of the word. BitCoin may not be the most popular option to trade with at a given time, but unless the internet is wiped out completely it will remain trading on some market somewhere and will always serve some purpose.
It could be replaced by something better, or just wiped out by concerted government action. Who knows. Not saying it will be, but in the time it takes quantum computing to go mainstream, we will probably have other things to worry about, like the planet dying.
