... Then we get into the possibility of power from nuclear fusion. I know it's one of those things that is always 'a few years away' - but cheap energy (free? unlimited?) is getting closer all the time.

I think this kind of highlights the fact that technology is racing ahead of us, and it won't be long before it goes way beyond the ability of mere humans to understand, and it is computers themselves doing the thinking and theorising. The most promising form of defence against quantum attacks currently under development seems to be NTRU, which relies on some quite esoteric lattice-based maths which is frankly already beyond my ability to comprehend. In a few years' time I can imagine that it is computers themselves coming up with these models, and humans struggling to keep up. It's an infinite arms race into the distance really. So long as there is one side attacking and another side defending, it's difficult to see where it will stop - unless there is some fundamental facet of quantum mechanics that provides a final barrier to one or both sides.