I have a problem with that paradigm. Firstly, if that is true of rangers, it certainly explains a lot, since they only get 4 subclass abilities, compared to 11 or so class abilities. If the majority of their power is supposed to come from those four abilities, I find it completely unsurprising that the ranger typically comes across as under-powered. Especially before they get their sub-class and in the gap levels where they aren;t getting new subclass abilities.
As for the "subsystem", well, first of all, there is only one, so you are talking only about spell casting because we don't have another sub-system to work with.
But, Druids, Bard, and Clerics also have very nice and impactful abilities. Clerics less so than the other two in general, but it is there. And these abilities (Bardic Inspiration and Wildshape particularly, as well as subclass features like the Dream Druid healing pool or the glamour bards charm abilities) change how the class plays almost as much as their spell choices,.
The other substyem is the skill system and this is how the rogue stays strong. Heavy interaction with ability, skill, and tool, checkis where rogues get their power.
Same with the full casters. Their poweer comes from the spellcasting system itself. Their other features like Wildshape or Bardic Inspiration are toppings on topping.
If you downgraded full casting on the bard, cleric, and druid then made the "warrior" subclass features base class features, those classes downgrade. A half casting druid who gets both mood and Land features is weaker that a full casting druid with either Moon, Land,or Shepherd.