The quantum discussion is very interesting.
To provide an easy answer, the solution to the quantum threat is quantum encryption. But this is the chicken or the egg since the first quantum computer that could break the current encryption could be used to clean everything out before a new quantum encryption is in place.
There is one caveat: everything would be broken. Would bitcoin be the first target. All government and corporate information would be available.
Do you target Bitcoin, Google, national security agencies, or the banking system? Everything would become one's playground with that.
The NIST has already recommended 4 quantum-resistant encryption algorithms and US agencies are currently upgrading their systems... so I think national security agencies should be relatively safe. I imagine the financial sector has more money to play with than government agencies, but I'm not sure how far along they are with upgrading to the NIST-endorsed algorithms... but you're right, quantum-hackers might only need a couple of financial institutions to be vulnerable to be more profitable than Bitcoin. I guess the other issue with Bitcoin is that the price would drop dramatically if they were to flood the market so the profit might not be as vast.