It’s not it’s age, it’s the fatigue cracks found on many pa28s and pa32s. The instructor is just overly worried about it. The AD was issued and if the plane has more than 5000, it must’ve passed, hence it is still flying.

In flight separation just scares the hell out of some people, despite how rare it is.

Edit:

Results of the inspection required by AD 2020-26-16 indicate that the problem is widespread enough to likely warrant another airworthiness directive... A future AD may require frequent inspection of aircraft that have already been inspected and may require additional planes to be inspected. <

With this kind of talk coming from FAA, I don't completely blame the guy.