I've reached 35 MPH downhill under pedal power; if I had wiped out I would have been in a world of hurt, or dead, depending on how it had gone.
Two days later, tooling around on flat ground, my handlebars came apart in my hands, thanks to hidden long-developing stress fractures, a characteristic of aluminium alloys. (There'd been numerous previous falls.)
Speed is indeed fun. But its consequences can be extreme.
(I've hit 80+ kph / 50+ mph on other rides.)
Another factor is rider mass. An elite cyclist at 60--65 kg (135--145 lb) can brake far more effectively than a past-their-prime 115--135 kg (250--300 lb) rider, particularly, again, on a grade. Brakes have only so much stopping power.
E-bikes make higher-speeds for less-experienced and less-skilled riders far more attainable.
Typical recreational cyclists spend most of their time in the 20--30 kph (12--18 mph) speed range.
But you're right that even at 35 mph you'd be in a world of hurt. Every time I see a bicyclist going aggressively downhill in a bowl helmet and a barely-there lycra suit, I cringe so hard. You know how they get rid of road rash? They aggressively brush it out with what amounts to sandpaper.
Wearing a properly fitted helmet, I was knocked unconscious and concussed quite badly by the hit to my head.
Bike is not safe. And it is not even about being hit by the car. Cyclists often fall and often it results in broken bones etc.
And "depend on people to do the right thing at the right time with the right skill" should be the absolute LAST fallback - only used when there is no other reasonable option.