If quantum computers were actually available (at least to someone) and were effectively able to successfully attack cryptographic algorhitms, there would be a lot more profitable ways to use them; and, if one really wanted to use them against the Bitcoin ecosystem, cracking private keys would be a quite more useful target than mining.
The profitable way to use these kinds of technology would be to simply mine altcoins on a multipool, trying to do anything else would likely cost more then it would generate in revenue.
But we are not here to talk on how make them profitable, but if they were a good resource to destroy Bitcoin.