My only thought is that they should have put more care into designing subclasses progressions so that their levels were better placed in the 20-level frame. (Additionally, IMHO they should have generally made subclasses at least 5-6 levels worth, so that at the minimum they would account for 25% of a character's identity).
Instead, too many people got hooked on the idea that, since it's not necessary to strictly adhere to design constraints (like 3e too often did), then not having any logic at all must be a good thing... but what seems like it's not a problem now when 5e is fresh and new, will become an annoying feature later.