If bitcoin remains invulnerable to them, then it is possible that the advent of quantum computers will even be useful for bitcoins in the sense that it will be possible to return to circulation over six million bitcoins that are now irretrievably lost.
1+ million Bitcoins, the other "lost" coins are QC resistant
The other "lost" coins are "first generation quantum computers" resistant. "Second generation quantum computers" that include hashing algorithms will get them.
I have really learnt so much from this thread. I feel like the discussion now needs to heads towards Mining. How Quantum Computers could affect mining
The first quantum computers won't be able to mine Bitcoin because they will not have enough qubits to get the hash of the next block. For that task 2^128 basic quantum operations are needed. That is something for the "second generation quantum computers".
But to get the privatekey only 128^3 basic quantum operations are sufficient and will be within the range of "first generation quantum computers".
https://en.bitcoin.it/wiki/Quantum_computing_and_BitcoinAnd for the "second generation quantum computers" people are already developing post SHA-hash signature systems. So we would then change to post SHA-hash signature systems before "second generation quantum computers" exist.