I think it's really hard to make DM content that is broadly applicable.
Things like random tables are useful to certain styles. Other things like advice and optional rules are cool and useful, once. But you never really revisit them. Maybe world lore books? But my favorite books as a DM are monster books.
There was an Atlas Animalia from Metal Weave games that gave like in-depth lore on a small subset of creatures. Really niche stuff like ecology and habitat, behavior, almost like an article on a real creature. Loved that book. But I doubt most DMs value that content like I do.
I don't know what WotC can make for DMs that is broadly useful to experienced DMs. When players it's easy, a new subclass or race.