Your argument comes down to - or comes across as coming down to - two forks:
(1) An argument from popularity. Not a good look, and certainly not a sound support.
(2) An argument from "kids can read the dictionary" - this is... well, let me put it as below.
There are hundreds and thousands of books you can get that introduce you to new skills - learning how to play piano, learning how to code HTML, learning how to bake, and so on. None that I've ever seen is structured the way the DMG is structured.
There is something very telling about that. What's more, none of them assume that just because you can read a word in the dictionary means you
sufficiently grasp the concepts involved. Suffice to say that between your say-so and the work of professional pedagogues on how to write and structure an instructional manual - I find the latter group to be far more compelling.
Now, if the DMG's purpose as a book is
not intended to help new DMs learn how to run a game, and is instead intended to be an all-purpose reference manual for running games - similar to way, say, the rulebook for
Axis Empires: Totaler Krieg is not intended, as such, to help new players learn how to play the game - that's all well and good, as far as it goes. But if that's the case, it should be made explicitly clear in the DMG so prospective new DMs know that they should use a starter set or other resource to learn DMing skills, and use the DMG as a reference.