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Sure, if the thing starts burning oil due to leaky rings or valve seals things are going to go downhill, but in general with a fuel injected car which has been maintained well you will not see any significant change MPG or emissions.
Which is part of my point that ICE cars get dirtier as the age. Sure CARB and some other states limit the amount of allowed exhaust annually but where is that burning oil you mention going to? What about the ground oil that is on parking spaces and roadways? That did not stop when Smog checks began. Dirty is dirty.
Lets look at all the other ways ICEs continue to get dirty. Have you been to Mexico? What if they drove all EVs there and the rest of the world? What about the worldwide millions of non smog checked garden tools, motorcycles, forklifts, construction equipment, golfcarts, etc., etc, all which would be clean on day one and day1000 if they were all electric and not gas, oil-gas mix or diesel from day one to dirty 1000.