abirdcall
(she/her)
There's quite a lot of stuff already for 5e that ends somewhere between level 10 and 15. One of the main complaints I've heard about the product offerings is the lack of high-level stuff. So, it's nice that they're scratching that itch somewhat.
Now, you might argue that a dungeon is perhaps not the best setting for epic-level stuff (the old BECMI version went dungeon -> wilderness -> settle down -> planes, with each stage including the previous ones as well), but it's definitely something.
I think an 11-20 book would be fine. I wouldn't buy it but it would be what the people who want high level play are looking for. They don't need more 5-10 stuff, but I'd like it.
1-5
5-11
11-20
In 3 books would be awesome.