I agree completely. If someone develops a quantum computer that can break existing encryption with ease, then there are much bigger targets than bitcoin. As mentioned by PrimeNumber7 above, if it becomes public knowledge that someone has hacked bitcoin and stolen say $1 million of coins, then crypto will take a nosedive and that $1 million will fall in value very very rapidly. If they steal $1 billion, then I'd be very surprised if they could cash it out to fiat before it lost most of its value.
We all know how volatile crypto prices can be, with even the merest suggestion of a rumour of bad news often enough to cause the whole market to tank. Something like a quantum hack would have a huge impact. If this hypothetical malicious actor with a quantum computer wants to make a huge amount of money, they could go after banks instead - that would be much more lucrative and probably easier. And if it's a government doing the hacking, then again it would be much more advantageous for them to hack a rival government (US vs China for example). They could wreak havoc, with infrastructure a likely target, but in theory any state secrets or corporate data would be vulnerable.
Finally we must also remember that one of the best things about crypto is that good coins are under continuous development, and defences against quantum attack will likely be in place long before it becomes a real risk. These coins are developed by some very smart and very tech-savvy people. If quantum computing becomes a threat, it won't take these people by surprise.
As I've mentioned before, I think that whilst a lot of work has gone into building quantum-resistant systems using classical computers, one of the best avenues of investigation is defence using quantum computers. There has been plenty of research into various methods of
Quantum Key Distribution, and this research continues with approaches such as
Kaks three-stage protocol. Perhaps this will be quantum-attack-proof, or perhaps not. But the key here is that defence is actually moving faster than attack.
I get your point that you and primenumber7 are putting across although lets not forget that Bitcoin has already had some very serious bugs in the past which involved basically printing off Bitcoin. This was a big thing at the time and luckily wasn't abused. You would think such negative press would have destroyed Bitcoin but it didn't. If money was stolen then Bitcoin would take a dive but I wouldn't say it would be the end of Bitcoin. Cash is stolen everyday and fiat currencies gets printed off fairly regular but that doesn't stop people using it. Bitcoin has its strengths and god forbid we will probably have incidents such as the earlier issue with printing off Bitcoin. I use printing off as a comparisons but really you could double spend coins and keep them and therefore create Bitcoins out of thin air.
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.