You misunderstand the statistic. The 90% compliance measures cars that were driving into a future LEZ area. There was no need for any of those vehicles to be compliant at the time of measurement. The measurements were performed to analyse the impact that the new LEZ would have when introduced.
Additionally, I think it’s extremely misleading to say that you need to buy a new car every eight years. In this case, you might need to buy a used car that is already eight years old (or much older if petrol). They are not expensive compared to new cars. In the case of London there has also been a scrappage scheme to assist people, though this probably isn’t available everywhere.
There is evidence that 20mph zones do save lives and injuries even if people don’t obey them. This is because they still drive slower than they would in a 30mph zone.
I would agree, however, that if the limit is set to 20mph then the road design needs to be changed to match that, making it uncomfortable for drivers to exceed the limit. Unfortunately the UK is quite institutionally poor at this kind of traffic calming design compared to some of its neighbours.