Maybe I'm one of the few people who DON'T want the DMG to have this whole big section on
"How To DM". Because quite frankly... learning how to DM and what to focus on while DMing is such a personal and personal preference kind of thing that I in no way wish to see a single way proscribed as "here's the way to do it."
I've read how a lot of you people run your games here on EN World and know for a fact I would NEVER want to play in a lot of your games, LOL! If the DMG was then giving us instructions on how to DM in the styles that some of you DM? That would be horrible for someone like me! The last thing I want is for more people to think that
your ways of DMing are the "proper" ways to do so, heh heh! We've already got worshippers at the altars of The Alexandrian and the Angry DM... to have any one of their styles of DMing possibly become the "instructions" of how to do it in the DMG? No thank you!
And by the same token... if the DMG was to be all about MY methods (and folks like Matt Mercer, Matt Colville and the like) of narrative and story and throwing out the "board game rules" of D&D combat and mechanics even more than it does already (with the 'Rulings, Not Rules' mantra)... I think a whole crap-ton of you folks would absolutely FREAK. You wouldn't want new players to learn to treat the game in my way either, because that would basically stomp out your preferred ways of playing the game over time too, players thinking your ways are "wrong" because those aren't what the DMG has taught them.
Learning to DM well is a personal thing that you only get better at and figure out your particular preferences as you do it. Thus I am wholeheartedly in favor of what we have... which is the Starter Set (of whatever type it is) showing players the very generic and
baseline instruction of what the DM needs to do to run the game... but any additional
instruction on "how to do it" should be left to the DM's own devices, or websites, or YouTube videos, or whatever. Let the DM who wants more info go out searching for it on their own... rather than proscribe a single way to do it in the Dungeon Master's Guide.