Faolyn
(she/her)
They thought about doing that with Level Up (combining the PHB and DMG, not the MM), but they still needed to split it into three books. As it is, the Adventurer's Guide is over 600 pages. Fortunately, the second book, Trials & Treasures, consists only of magic items and exploration-type stuff, with the actual DM stuff being stuck in the Adventurer's Guide. If spells and magic items were put into a single volume, they might be able to combine everything else.I think the DMG suffers from the Jack of All Trades issue - it's supposed be a DM's primer, a toolbox for adventure creation and optional game components and a host of reference charts & information (such as magic items). It's either going to have to drop something or be a (much) bigger book. I kind of hope it gets chunked up to 1E AD&D size myself. Though I wish they could move magic items to the PHB, for example, but its already cluttered by a hunk of spells. Or rearrange the books into PHB, MM, DMG and a "Treasures & Spells" book. God knows the latter has enough stuff for a book of its own, but buying 3 books as is already is a bit of an annoyance - and going the PF2 way is madness with trying to stuff 3/4 of the game into one book.
<Edit> On that last thought, maybe just a PHB and combine the DMG & MM into one book?
Unless the monsters were pared down significantly, there's no way they could put them and everything else into one book. It could be done, but not in a way I think would make people happy. You could do things like, have a single dragon statblock (well, one statblock per age category), with templates and descriptions for each of the species. Do the same for the humanoid bad guy statblocks (commoner, warrior, lieutenant, warlord, casters, assassin, trapmaster, beastmaster, etc.), likewise with templates and descriptions for each species, since let's face it, stat-wise, there's very few mechanical differences between orcs and hobgoblins and kobolds and whatever. Ravening Beast could be a set of statblocks (one for each tier of play), and templates or just lists of traits you can pick and choose from to make worgs, griffons, owlbears, chimeras, manticores, or whatever you want. Giants could be divided up much like the humanoids, with differing levels of size, hp, AC, and attack bonuses to make them Large, Huge, or Gargantuan. Do something similar for fey, undead, and beasts, and you have something like 80% of your monsters tackled in a section probably half the size of the original MM. Or you could simplify it even more and just have very basic statblocks and a list of traits you can mix and match for all of them, so if you wanted to, you could end up with dragons or goblins that could turn people into stone.
It just wouldn't be something that could be run right out of the book, which would annoy people a lot. OTOH, using online tools would probably make it a lot easier to make the monsters.