I should be able to go cycling as an everyday activity in street clothes. Most people feel they cannot do that as it is perceived as too dangerous. There IS evidence of that, and it is incorrect to state that mandatory helmet laws are not a significant contributor.
I'm struggling to follow your argument here but ...
You can go cycling in everyday street clothes (with helmet). Specialist cycling clothing 't really protect you in the case of an accident - compare it to specialist motorcycling clothing.
Arguably an everyday pair of jeans provides more protection (and privacy) than spandex.
The only down side of cycling in street clothes that I'm aware of is the clothes aren't designed for cycling and may wear faster, and you need to keep the away for the chain.
I think that the perception (if you'll pardon the pun) is that a cyclist nowadays *need* hi-viz stuff. And perhaps strobe lights. And probably quite rightly... most cycle injuries are caused by motorists, and a fair chunk of those by motorists who didn't see the cyclist.
Give it enough time and these would also be mandatory. After all, exactly the same justifications can be applied as for helmets. And I'd hazard a guess that a hi-viz jacket and a strobe light or two would do more for a cyclist's safety than a helmet. And it's far cheaper than separating motorists, who are the root cause, from cyclists.