> If quantum computers do in fact result in a couple of thousand orders of magnitude raise in the hashing rate... well then miners will use different hardware the difficulty of the network will adjust each 10 minutes as new hardware gets added, and after the change you'll just have a large army of quantum miners instead of ASIC miners.

If it's a couple of thousand orders of magnitude, the first person with a working quantum chip might entirely dominate the hashpower, long before the chip is available commercially. So you'd end up concentrating hashpower into a couple of BigCorps's hands. Like, if I went back in time to the beginning of BTC with an ASIC I'd probably completely dominate the baby blockchain and outcompete the entire network.