There isn't much of a threat IMO as even the industrial-grade supercomputers won't even be able to crack a private key off of a bitcoin address. Also keep in mind that there aren't any commercially-available supercomputer anywhere that can come near the level of quantum computing, or even quantum computing in itself is still an exclusive privilege to experience. Again, it will potentially take hundreds of years before we crack SHA-256, before going in to the juicy tidbits of bitcoin's algorithm. Perhaps by then, humanity would cease to exist because of reasons..
Lets take your example of quantum computing and see what we get. Quantum computers already exist but work so hard that cooling is an issue, but once that problem is solved= watch-out. Here is a statement from bitcoin wiki "it is known for sure that it would take a sufficiently large quantum computer on the order of only 128 (to the third power) basic quantum operations to be able to break a Bitcoin key using Shor's Algorithm. This might take some time, especially since the first quantum computers are likely to be extremely slow, but it is still very practical". That works out to 2,097,152 operations. Now imagine how that number will change once we actually learn to harness the true power of quantum computers.