Any vehicle on the road has a legal obligation to operate their vehicle safely and lawfully — that means a bike ought never be unable to stop in time.
Some person could get horribly injured due to a bicyclist failing to control speed.
Would you make the same argument about leaving unmarked construction equipment behind blind curves on the freeway?
Blocking or unexpectedly merging into downhill bike lanes in the city is similarly bad.
I have. For close on fourty years of much cycling and driving. Which may be a contributing factor in my never having suffered an accident.
Well, not necessarily.
If your reflexes are good enough, and your brakes are good enough, and your tires are good enough, that you can safely stop a bike going 20mph on a wet downhill section, then yes; you are still in control of your vehicle and operating at a safe speed. No legal problems there.
But maybe your reflexes aren't that good because you haven't had coffee yet. Or maybe your brakes are worn down. Or maybe you're on crazy thin road tires that have effectively no traction going downhill in the rain. Or some combination of all three.
The point being, if it's not physically possible for you to stop that bike in whatever time interval is required, are you really operating that vehicle safely? Probably not.
And to the extent that you're legally required to operate at a safe speed and maintain control of your vehicle, your behavior would no longer be "perfectly legal."
So 20 mph + downhill + wet roads may very well be illegal for some combinations of cyclist and bicycle but not others. It seems premature to declare that perfectly legal in all cases.