This was discussed a thousand times. If quantum computers are ready to crack today's encryption algorithms, then everyone and everything in the digitalization world has a problem, not just crypto currencies. Until then, there will be suitable quantum-resistent algorithms, which will then be implemented in the software. So, no need to worry.
When quantum computers appear, we will know what will consequently happen with human civilization and digital world. For now, let's enjoy and use amazing and convenient services from Blockchain technology. We should not seriously think of quantum computers too much when it is just appear in theory for now. When it appear, it means technology will move to higher levels than we have now, so on that day, human civilization might have something stronger and better than current cryptography. That's all a story of far future.
As for quantum computer, I think it will be invented much earlier than you think .. technologies are very quickly improved, if you look at the evolution of a mobile phone - gadgets that we have now - 15 years ago, no one could predict this progress. I think that a quantum computer can be invented in the next 10 years, but you are right - now there is no sense in predicting what will happen.