You're assuming that we have arrived at the final solution, which is not the case. The proof of work could change as required, even if it's gradually over 10 ye

You're assuming that we have arrived at the final solution, which is not the case. The proof of work could change as required, even if it's gradually over 10 years to not weaken the network by making current, specialized hardware useless overnight, there is nothing that would stop such a move. If the power consumption became a serious problem, miners would probably consent to modifications to the proof of work algorithm, which is all it takes. And changing over a course of 10 years (there's an incentive to constantly upgrade hardware anyway) is probably still way faster than the banking system would change on a global scale if it was running into a problem. While this is a different story because hardware and the PoW algorithm is involved, Bitcoin has a story of "breaking" changes. There will be more with the increase of the block size, etc. So I doubt this would be aniticipated way before it made it's way into the protocol, so the affected parties could prepare as well.