We shouldn't prohibit dumping toxic waste in the river, we should just tax it!

I am familiar with the EU situation. The carbon tax you would have needed to achieve the effect of fleet emission standards would have been political suicide.

And that is not just psychological. People who buy used cars and drive their cars until they fall apart are well correlated with people who can't afford high carbon tax. Buyers of new cars are the people who can. Carbon Tax would mean massive redistribution of the money raised. Yet another political mine field.