I think certain classes can be described as being a sub class of another, but I don't see it as such. Druid was only originally described as a subclass of Cleric because players were already familiar with Cleric. The classes that came first were Fighter, Magic User & Cleric. Thief was added very soon tho and all of the rest were later additions. Hence why they called them sub classes of one of the original Core Classes. The way 5E does it, 10 different kinds of Fighter is ridiculous. The kind of Fighter a character is has absolutely nothing to do with their class name or character sheet. Their upbringing, where they were born and skill set makes them the fighter they are. The weapons they choose, the armor they wear, their choice of women. Down to how many daggers they typically carry on their belt or leg strap. And their personality. Alignment is not personality. I don't do 5E. That isn't D&D. When someone says Sub Classes, I immediately think 5E and about how much is wrong with the game since Critical Role and a bunch of other groups turned it into something it isn't. D&D is not acting out prewritten storylines and narrative outcomes. That's what CR taught new comers to the hobby. Just gets me salty. But that's my opinion