You seem to forget that the DMG isn't for just old timers who know how to GM. It's also for newbies who are just learning to game. As well as DMs who may know how to make NPCs but not know how to make the game fun, etc etc.In most every edition of D&D, I have used the DMG very little. Mostly for the treasure tables and magic items. Make this book earn the money we pay. If it's advice, you'll probably read it once. That is better suited to online articles. Just make sure the book is very good at pointing you to the right place online. FWIW, I should use the bulk of the book more than once. A third of the book on creating your own game elements (classes, races, spells, etc.), a third of the book on world-building with examples that can be pulled and ready for use, a third of the book for NPCs showing how to make them useful and memorable plus plenty of examples across the levels, a third of the book for crafting story including instant hooks and plot element tables, a third of the book for treasure and magic items.
Piecing it off to online articles means that the majority of people will not see it.