Yes, post-quantum cryptocurrency can be built using Lamport signatures or, I think more likely, a full-fledged post-quantum public key system. But would such a hard fork still be "bitcoin"? Will there be enough coordination to make the jump? Why bet on it? Added: in other words, you have now switched to an argument of the form: this community will respect property rights, which is exactly opposite to the technical argument you started with.

Also, it's not just about Shor's algorithm. Grover's algorithm is a big deal. The advent of quantum computers will dramatically concentrate the pool of hashing power into few hands. I'm not sure what will happen, but I think that there is a good chance that the value of existing cryptocoins will be wiped out, even though the technology will be resurrected after quantum computers become widespread.