I think it actually makes a lot of sense to make "having a PC class" a relatively rare thing in a given population. If you were careful about your metaphysics (and gave more attention to class design) you could make whatever it is that allows a character to grow that way be a known, limited in-universe factor. "Protagonism" as an inherent trait.Personally I think this is fine. Characters are gifted, and whilst some NPCs are equally so, some might not be and would have just learned some of the class features etc. For example, I have no problem with a temple pries, who has cleric's casting capabilities but have not actually ever bothered to learn weapon or armour proficiencies, as they simply never had need for them.
And not all NPCs need a class. Some can just be basically commoners with perhaps an extra skill, proficiency or a feat. It mostly makes sense to think more powerful people in terms of classes.
I miss demographic tables, even if they were never particularly well thought out. I'd love to know just how many characters of various levels/classes to expect in a given population.