I've never really understood the argument that quantum computing could break Bitcoin. Even if it did manage to crack one address successfully, that is just one address...there will not be a stage where it can suddenly crack them all. To be able to start brute forcing thousands of addresses, you have to do hundreds first...and before that, tens, and before that one....and to do 10, you'll need to put it 10x the computational power than one...and if a quantum computer can't even crack one yet, then how long will it take before it can do 10? That's my train of thought anyway...and it completely disregards if the core devs will push an update to make Bitcoin resistant if the day ever comes where one is successfully cracked by quantum computing.