I don't think that's what people are implying, just that the DMG/PHB onboarding could be improved. If you want to learn how to play, and part 1 of the book that the title indicates is supposed to do that is about how to create your own Gods/World/Government, that's not the most approachable introduction. As you indicate, people are learning to play, in great numbers. Some through watching videos of other people and winging it, some through friends, some through the starter sets, and some, like me, through buying the rule books and learning that way. The people who are arguing for a better DMG for starters just want that last option to not be hindered by organization (and to learn from other texts that do it better).
I, personally, was worlds more comfortable having read the PHB and DMG, and then playing through LMoP. There was much that came up in the course of play that was not covered in the adventure book or the basic rule kit included. I was more equipped to handle it, and had a much better game, because I was prepared. Based on my experience, I would not want the starter kit to be my complete and total introduction to the game as a DM.