They can keep using whatever they're currently using.

This doesn't take away anything. It's a new standard.

Based on your argument, one should add new safety standards to cars like seat belts, because old cars might not have them.

one shouldn't*

One shouldn't add new safety standards*

writing before coffee...

To be fair, some municipalities do actually require old cars to be fitted with seatbelts... air bags, not so much, because apparently changing a steering column is too hard?

Well, I just noticed I said 'should' and not 'shouldn't' which I wanted to say

But to your point,

While they might not be required to retrofit, one shouldn't stop defining new safety standards.