Good God!

Quantum computers can hack bitcoin. They're just not big enough yet. When they become big enough they will be able to hack most conventional cryptography ... this is an issue on a scale that completely dwarfs cryptocurrencies, so:

1) if/when quantum computers approach the size to crack bitcoin, we will all see it coming years and years ahead of time
2) you can be sure that if it's possible to protect from quantum attacks, e.g. by having some small contribution from quantum hashing power on the network, then it will be implemented in time
3) this is not even a bitcoin issue, it's an everything issue, so it's like worrying if worldwide all out nuclear war will disrupt the bitcoin network ... yes, probably, but nobody will care