New laws can certainly be made retroactive; for example you can't drive without license plates just because they were not required when the car was new. But in most cases, existing cars are "grandfathered".
TS13571L did not have the holes for the mirrors when it came to me, and I drove it some that way before deciding to drill the holes. I definitely missed the mirrors.
One place where I really like my wing mirrors is in traffic where the SUV (or semi) behind me is too close to see around otherwise, and I want to change lanes before he gets high on my exhaust and forgets I'm hiding under his front bumper! An old whiplash injury makes it hard for me to crane my neck around to see, but the wing mirrors let me look "around the corner".
YMMV, of course.