Not all QC is alike. The slowest type wouldn't effect us much at all, the best type would make the concept of any type of currency irrelevant.
Bingo.
You do realise how nutty you sound? Your evidence of the existence of working quantum computers is that that there is no evidence of working quantum computers?
No, I think it's nutty to assume the opposite. And by the way, your assumption that "there is no evidence of working quantum computers" is dead wrong since scientists have already laid claim to the existence of working quantum computers the size of a handful of qubits, though they claim they aren't very useful in their current state and can only use them to solve extremely basic and not very useful problems. I'm simply giving three plausible scenarios (there are others, of course) and one of them is likely true:
1) There are scientists that have built rudimentary quantum computers, but they don't know of others who have built vastly more complex ones.
OR
2) There are scientists that have built extremely complex quantum computers, but they are minimizing their results to the public.
OR
3) There are scientists that have built rudimentary quantum computers, are aware that others have built vastly more complex ones and are trying to replicate this complexity, but are restricting this information to the public.
You and others are taking the position that it sounds nutty based on the arrogant assumption that you think you're somehow privileged enough as another human being to know exactly what's going on. I'd bet you're also a person who thinks it's "nutty" for people to think that extraterrestrials exist because you/we haven't seen them despite clear evidence of a hierarchy of life and intelligence, and to assume that that hierarchy stops at planet Earth is one of the dumbest, most egocentric conclusions a person can reach. It's hardly different than the assumption proven false long ago that the sun revolves around the Earth.