I agree - beginning to condition mask mandates on not being vaccinated opens a door to very difficult enforcement. If, for example, airlines decide vaccinated people don't have to wear a mask, they're now asking flight crews to be aware of which passengers aren't vaccinated and enforce the mask rule on those passengers. Seems an unreasonable ask for crew members who have enough to do already - and it opens the door to far more bickering from passengers.
It's just not a good idea.