If this ever happens then I presume the entire internet will be compromised for all of us.  Bitcoin is a financial motive for such a computer to be created.  But by breaking encryption, there are a TON of other things to break and steal.  Such as government information and other things.  Bitcoin would become useless at least for a while, clearly.  Why steal something and render it useless when you can steal US, Russia, China intel and sell it?

In my opinion, we simply can not function based off this 'what if?' fear.  For all we know, North Korea may be preparing to show the world for the first time something more powerful than what any other scientists have ever created, a computer that breaks encryption way before the known most powerful quantum computers can.  In fact, it would be an advantage to Kim.  Yet, living in this fear is no good.  Because what is the point?

Considering this is an issue already being worked on, I believe we are on the right path.  Things can happen along the way, we simply have to accept the facts and move on.  Bitcoin is digital after all, it is definitely prone to a few attacks.  And on the other side of the blade, you have yet another risk.  What if a better currency comes ahead of Bitcoin proving it can do things better than Bitcoin can, including being bullet proof in front of any type of computer known to be concievable to man?  What do we do then?