.......if said instructor is very much part time and can afford to survive without what would be relatively little extra income, or only flew with aforementioned PPLs, I would see that as being outside the spirit of the law if not illegal.
That is nonsense. A Flight Instructor is a professional. Even the smallest amount of flying and earnings could be important to an individual. Who are we to judge? We must comply with some very silly, out of date and some very unfair regulations at times without various people making up new ones. If any professional needs to renew to work then they should do so but, of course, safely and according to current regulations as they are written.