Not many people will do >50kmh even downhill (even with disk brakes), and on the few occasions I've done so, I absolutely would not say there are "significant controllability problems". In fact, quite the opposite.
[1]https://www.youtube.com/watch?v=FN6Z8UgVOuw Nb - these were shot in the days before the 'supertuck' (sitting on the crossbar) was banned.
Who on earth is using ordinary city bikes at 50km/h near cars? Even class II e-bikes are limited to 20mph/32kph.
Class III are limited to 28mph/45kph but are forbidden from bike paths.
[edit] 50kph under your own power is the average speed at the Tour de France.
My bike was quite stable at 40mph.
No I wouldn't go that fast on a bike path, this was only due to the power of gravity.
Now, I fully agree with the spirit of your post. Even the spots I'm talking about are short bursts.
(Why am I using >50 km/h as the benchmark? Because that's the sort of speeds you easily end up at if you try to keep up with cars downhill.)
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Edit: it seems that people are missing critical words in my comment:
- "Ordinary upright/city bike" means "not road/touring/etc bikes". They are constructed differently and comfortably do well over 50 km/h.
- "Downhill" means significant gravity assistance for a few seconds up to a minute, and not "sustained under pedaling power alone". I'd be impressed if you regularly did 50 km/h on the flats with one of those bicycles.