I'd agree that no limit will improve the skill of bad drivers, however if we imagine a scenario (whether we agree with it or not) where 100% of vehicles in a 20 zone didn't exceed 20, I'd assert the area would be safer overall for drivers and pedestrians. What happens at present is some drivers stick to 20, some (like me) hover around 25 but quite a few will do 30+, and a few of those will be nearing 40. This creates challenges such as drivers sticking to ~20 getting irate drivers (who want to go faster) tailgating them and pedestrians not having the certainty that all vehicles are moving slowly.20 mph schemes reduce accidents but dont improve driving, its quite possible to drive dangerously at 20 through poor manoeuvring, pulling out without looking, or just relying on other drivers to be careful on your behalf. IME the most considerate (urban) drivers are tradies in vans, the least considerate are middle aged women in Q7s on their way back from Waitrose or dropping their kids at private school, they think their motor gives them automatic right of way in every situation.
Like many things, discussions on this will go round and round because we hold different opinions, which is fine, however we're unlikely to coalesce around one pov.
Which is why, eventually, we'll be limited speed wise by the tech in vehicles.