There's another parsimonious, Occam's razor-favoring interpretation of the discrepancy between 5e and the OSR "DM crisis", although it's not going to be a popular one, especially with OSR players. Maybe there's tons of DMs in the OSR and not in 5e because nobody (not literally, but relatively) wants to play OSR games, so all the people who want to DM them can't find players, who are looking for something else to play. Personally, before people resort to some other explanation, I think that null hypothesis needs to be falsified. And "everybody in my group loves OSR games" doesn't falsify a null hypothesis.
I would qualify anyone who is playing something other than 5e, especially osr and indie games, as a ttrpg enthusiast. There's a smaller pool of people to play with, but I'd expect those who are into it to be into both running and playing in games.
Larger games like Pathfinder 2e and Call of Cthulhu...I don't know. Being a keeper for CoC is a lot of work too, which can be somewhat lessened by purchasing fairly expensive and expansive campaign scenarios, so in that way it shares dynamics with 5e. Pathfinder, meanwhile, has a lot of player options, like 5e; does that mean there are fewer people willing to DM?
So... what topics do people think would constitute useful "DM Advice" for D&D specifically? Not the advice itself, just the topics, like what would the chapter subheadings look like?
Honestly, for 5e I think the sly flourish products together contain the most useful advice. Earlier starter sets taught new dms how to make a dungeon, so the dm advice would include how to create whatever kind of scenario 5e wants you to create.