Well, I disagree in part for a few reasons.
1. Money. They will never stop making DMGs because, as you note, people buy them.
2. Tradition. I am not someone who demands tradition be followed, but D&D has always had the core three books- PHB, DMG, and MM.
3. Nattering nabobs of negativity. Yes, I think people will complain about the new DMG. But people always complain! It's modern life, and the internet. I mean, people have spent innumerable words, so many words that even I look at them and say, "That's too many words!" arguing about whether this is a new edition. Why do people fight? Because they can.
4. The DMG presents all sorts of cool things- from magic items to rules variants. I know that we all like to complain about the 5e DMG (um, the 5e14 DMG), but it actually had a lot of good stuff in it. Stuff most people didn't know was there, but still.
5. All that said, I agree that on-line videos and short "how to" instructional guides and starter kits designed for it are probably the best way to teach new DMs,* not the DMG. I think that the issue is that we need to stop thinking of the DMG as the guide to teaching DMs how to DM, and instead think of it as a resource for DMs.
*Well, the best way is to play and observe. And then to get reps as a DM, because you will mess up and make mistakes and learn from those mistakes. But specific and targeted videos, such as the ones WoTC already has made, are better than reading the DMG.