Rather than responding to individual posts (and parts thereof), it is probably clearly for me to state my own position, as logically as I can.
Generally, we have policing by consent.
Consent is best arrived at when "the law" is warranted, practicable, justifiable. When it "makes sense".
If it is not, it'll just get ignored by more and more people, until it becomes farcical (and a waste of time and resources).
The point of [cars, for simplicity] is to get from A to B.
If we wanted complete safety (from cars), we'd just outright ban them.
So we, as a society, accept a degree of risk, which is hopefully more-than-offset by the benefit gained.
The level of risk varies depending on a number of factors.
For simplicity, I'll assume (incorrectly, but for simplicity) that all drivers are of equivalent competence and safety, with regard to eyesight, skill, judgement, reaction times, etc etc.
I'll do the same for all cars too - same stopping distance capabilities, same handling parameters, etc etc.
And for all road surfaces - all equally well-maintained, all the same grip levels, etc etc.
And assume the weather is always fine, dry, and clear.
The variables now are (non-exhaustive list):
- posted speed limit
- width of roadway
- whether it is single or dual-carriageway
- presence of, width of, pavements
- presence or absence of side junctions
- presence or lack of bends
etc etc
As not all roads are the same, in terms of risk, not all speed limits can be justified as being equal either.
Enforcing a blanket "20mph" was doomed to fail, because most sensible drivers would see them to be unjustified in however many circumstances, and some of those would ignore them.
