the undeniable fact is that at some point something will come along (either out of the blue and shock us all or by virtue of a slow build up) to pose a serious threat to private keys.
Yes, perhaps. This is an important point to consider, and I do have a favoured approach which I'll get to in a moment.
Public key cryptography is insecure against a QC running Shor's alogrithm, whilst certain symmetric systems such as AES256
do seem quantum secure against the best QC attack (Grover)... and this holds no matter how many qubits you throw at it.
The key point in any cryptography is that it may be secure now, and it may be secure against such future technological or mathematical advances as we can envisage, but how can we ever say it's secure against such future technology as we can't even conceive right now? At first glance it seems we can never provide that absolute certainty. However I believe we can get close. This is where we have the distinction between
post-quantum cryptography, which involves using classical computers to devise quantum-proof systems and algorithms, and
quantum cryptography, which uses the laws of quantum mechanics to build a defence.
You will be aware of the Schrodinger's Cat thought experiment, where the cat is neither alive nor dead until it is observed, existing instead in a hybrid state, a superposition of both classical outcomes. Whilst this may be an absurd extension of the quantum realm into the macroscopic, it certainly holds true on a quantum level. The act of observation collapses the wave function and forces an outcome. This is an immutable physical law. And if we then combine this with quantum entanglement, this enables key sharing that in theory is immune to hacking or eavesdropping, because any attempt by a third party to intercept the key collapses and invalidates the whole thing. I'll go into it in more depth if the thread heads that way...