So, consider the purely hypothetical alternative...
D&D, in a fantasy setting, there are dragons and other magical monsters (including once-mortals who have crossed a line, like a lich or a warlock who has surrendered soul & free will to become a magic-wielding tool of some devil/fey/GOO/whatever), there are gods and other supernatural powers above mortals, and there are magical items placed in the world by such beings to tempt, bedevil, aid, or just mess with mortals. All PCs are mere mortals. Martial characters, much as they are or were in some preferred edition. Casters, OTOH, like 1e magic-users, must devote themselves to the study of the arcane/divine, from early childhood into middle age to even have a chance of gaining magical powers, lifetimes of study, conteplation, and/or prayer, with no time or effort spared to become proficient with mundane armor, weapons, or tools (caligraphy or something like that, sure), nor physical or social skills of any kind, in return, the learn the magical secrets available to mortals - knowledge of magic & suprenatural beings, including rituals that offer some protection from them or invoke them for aid (rarely answered) or bargaining (tho mortals have little to offer but their souls or servitude, which means no longer a PC), and how to use appropriate magic items. When such aspirants become frustrated with their lack of progress, they may well go out adventuring in the hopes of finding a magic item that may grant them the kinds of powers they seek. The ability to use such items also being a benefit of the class.
Imagine that's what D&D has been for, like, 48 of the last 50 years. How would fixing the caster/martial gap in that hypothetical scenario sound?