Both policies exist actually. For instance old cars and motorcycles are getting banned in Paris (outside of weekends IIRC). Even if as the grandparent points out making a new car generates a lot of pollution at least it would reduce the more local problem of having a heavy smog on windless days (and all the health issues that go along with it).

Banning old cars from inner-city driving is a nessesity. With how densly cities pack both cars and buildings, suspended particulate matter is a serious public health risk. But we can still allow them everywhere else where we aren't already struggling with keeping particluate levels at a reasonably safe level.