No real need to ban old cars, they tend to quickly phase out on their own. Heck, the state I live in has uninstalled all the expensive dyno equipment they used for pre-OBD2 cars because there just aren't enough left on the road to justify maintaining the test. They still sniff, because that's cheap, but not on a dyno.

Given that we still allow horses and Model T's on the roads (aside from freeways, due to inability of century-old cars to maintain that speed), I can't imagine why we'd bother banning them. The nostalgia is nice and their impact is negligible.