I would generally agree with that. If a game has a lot of specific classes, I think they should be more strictly diegetic and less reskinnable. (Although I think my reasoning there is backwards; I'd say that IF a game is going to feature strongly diegetic classes, THEN there should be a lot of them to make them specific to the setting.)
I think where D&D has gone wrong over time is that it's bound by tradition to include nondiegetic classes like Fighter and Rogue, which including more and more strongly diegetic classes like Druids and Paladins.
Either have a few generic, reskinnable, strongly moddable classes, or have a lot of diegetic, specific classes; don't have both in the same system.