> I'm entirely unconvinced that regular testing leads to an overall improvement large enough to matter assuming sufficient requirements were imposed on the vehicle at the time of manufacture.

Only ~95% of cars pass emissions tests last year (it varies by state). As each car is tested several times over its lifespan you’ll find the majority of IC cars eventually need something fixed to reduce smog.

This isn’t some wildly inefficient system it’s actually quite effective at improving air quality.