I'm sure that evidence can be assembled to demonstrate that a 20mph limit in towns will reduce both the frequency and severity of accidents. I am also sure that it is possible to enforce adherence to said limits by either camera technology or police presence.
Taken to its logical extreme any action which can impact adversely upon other members of society can and should be regulated in some way.
Environmentally we should limit the consumption of red meat, limit domestic temperatures, limit lumens per sq m to ensure energy is not wasted. Ban electric scooters as a danger to pedestrians, ban gambling, alcohol, tobacco as individual actions impact those around them. Ban sports for fear that sporting injuries will divert resources away from the prolongation of life at any cost.
Over regulation saps enjoyment and pleasure, and encourages the abrogation of personal responsibility. Regulation needs to be balanced with risk. Accepting some risk is a price worth paying to make existence something to be treasured rather than endured.
20mph limits - time constrained around schools which have lots of easily distracted children makes sense. Widening the scope significantly and ensuring it is robustly enforced is, IMHO, a denial of liberty, personal responsibility and choice.