The hypothetical person with the quantum computer able to break traditional encryption would probably not go after banks for multiple reasons. First of all banks would have definitely already switched to a quantum resistant encryption method and also the person who has access to the quantum computer will probably not be a ordinary person. They will be involved with the government or one of the wealthy elite in the world. Think facebook's CEO and while he may not agree with banks him attacking a bank mean prison. Whereas attacking Bitcoin is a little different laws are different and because you aren't attacking a centralized figure it is handled different. Also we need to remember that the likes of facebooks ceo would be in competition with Bitcoin as he is now releasing his own cryptocurrency. We do have enemies even more than the banks out there and unfortunate they are powerful enemies with seemingly unlimited funds.
I definitely agree with you that there will only be a few wealthy individuals that will have access to quantum computers and although I think its a good example that Mark Z would be a competitor against Bitcoin I think its fairly unrealistic in reality. Mark already has the marketing power and exposure that he needs for Libra and Bitcoin really isn't competing against him in that way. The only similarities they share is its a digital currency but as far as I know Libra isn't generated using encryption techniques and therefore can't be considered a cryptocurrency. Besides even if Bitcoin was a competitor I think the platform both Facebook and Instagram give him will knock spots off Bitcoins marketing techniques and he probably wouldn't have to invest as much into it compared to a quantum computer. I doubt he would have any other tasks to be completed with a quantum computer.