This argument is just treating everyone like children. If a 30 mph limit is seen by most to be appropriate but the problem is a proportion of people not complying then just enforce that.
By imposing 20 mph just to “slow things down a bit” then it just erodes away respect for institutions and the law.
What should happen is that there should be infrastructure changes to encourage speeds to reduce.
Enforcement means that the road is badly designed and means that the designers have failed, however the problem is that there's often not the potential to make the changes required to ensure that the roads are designed well.
For example, the ideal is that you have roads with bends in then so traffic can't go fast, however if the road is a Roman Road there's not a lot you can do to add bends in and so you have to look at other measures.
Sometimes it's as simple as changing the limit, for example I'd there's a village centre which had a 30 limit, changing it to a 20 limit will mean that people do go a little slower, however in doubt so it make the village centre more attractive and so people want to visit it.
Sometimes you do just need to annoy the drivers. For example, a new development has delivered a link road built to modern standards with good quality crossing points yet people are stuck in their ways and are going through a village because that's what they've always done, then there's a need to add in traffic calming which delays them significantly if everyone were to continue to do so to change the habits.
Once done the village becomes a nicer place to walk through and live in as most of the through traffic (the vast majority of which never stops in the village) has been diverted to the new road (which whilst longer actually may not add much to the journey time).
As with most things there are a small minority who think that driving is a right (it's not, it's a privilege and one that has a lot of responsibilities which goes with it) and how dare anyone suggest that they should be at all inconvenienced (even if that means that it results in others seeing a significant benefit, maybe even retaining their lives).
Arguably we should have a LOT more enforcement of the rules of the roads, as that would probably mean that quite a lot of people would end up earning 12 points on their licence. As actually the risk of getting caught breaking the speed limit, driving whilst using your phone, driving in the middle lane when you don't need to be, driving whilst over the alcohol limit and so on its very low.
Drivers generally get away with far more than they should, however it would shatter a lot of "good drivers" illusions of their abilities if there was much more enforcement than there currently is.
Likewise, there would be a lot of drivers who wouldn't be able to pass their driving test if they had to redo it (even if it was only to the standard they had to pass and not the more onerous standard that is currently required), there would probably be quite a few who never had to do the theory test or hazard perception who would fail them. It's ultimately the real reason that few support the idea of retesting for drivers, even though it would likely result in much safer roads.