Celebrim
Legend
The best start of this answer involves how the book was made, which was that that divided the dungeon into like 25 (36? Some number) sections, assigned a few monsters of appropriate CR to the sections until they'd separated out basically the whole SRD, and then asked a bunch of authors to individually work on the sections, sewing the resulting maps together like a patchwork quilt from some middle school arts project.
The result of this design by committee was uninspiring in both its grand design and its details. The individual sections were fairly monotonous with the same sort of monster reoccurring several times. There wasn't much in the way of exploration. There were few surprising connections to make, no sublayers to discover, no real sort of twists and turns to take beyond well, left/right. There wasn't a particularly cool overall layout to the map, nor was their particularly cool layouts to the individual map pieces. Thematically the whole thing was supposed to literally be some sort of dungeon to trap monsters in.
By abandoning the haven/delve format players I think lose any particular feeling of agency. It's just a labyrinth of left or right choices with random monsters stuck higgledly-piggledly in the way. And by abandoning verticality, you lose the ability to make hubs, shortcuts, and so forth to reduce travel time, to say nothing of just being monotonous to imagine.
Honestly, starting with the random dungeon generator in the back of the 1e AD&D DMG as a guide, I'd expect someone to develop a more fun and interesting mega-dungeon.
