> Draw a line, say this is for bicycles, pedestrians and cars have no business here, and bikes have no business being on any other lane as long as these exist.

This is the reality in many cities, if it weren't for the hopefully not surprising fact that people don't always obey traffic laws perfectly.

> Hey, we just solved 90% of the accidents.

No, you didn't. And restricting cyclists and pedestrians will not result in even small dent in the numbers of maimed or killed people in traffic. It's one mode of transport that's responsible for the vast amount of it, and that's the motorized one propelling several tonnes.

> and bikes have no business being on any other lane as long as these exist

And cars have no business being on other roads as long as highways exist ;)

This only leaves open how to enforce all of it without everybody shouting domestic terror.

If "shouldn't" worked we'd have no industrial accidents without any safety measures, no unwanted pregnancies and in general would more or less achieve heaven on Earth.

Unfortunately, the UK seems almost incapable of building usable cycle infrastructure (possibly excepting London). Your idea is just a recipe for magic protective paint and even more abuse of cyclists who don't want to be forced to use ridiculously badly designed infrastructure. e.g. Here in Bristol, we have an infamous shared cycle/pedestrian pavement along Coronation Rd that has a few trees completely blocking the cycle side which just means conflict between pedestrians and cyclists who have to fight over the scraps left over from motorists taking most of the space (https://www.google.com/maps/@51.4462522,-2.6064792,3a,75y,80...).