I dont mean to combine all three books into one book.
I mean, the 2024 Players Handbook needs all the rules to play a complete game of D&D.
We know the 2024 Players Handbook will have an expanded appendix for Beasts. These are mainly for Wild Shape, Familiar, Mount, etcetera. I hope they make nonmagical animal amalgums, like Owlbear and Griffon, into the Beast creature type.
But the DM can also use these same Beasts in combat encounters against the player characters.
The Player Handbook wont have every Beast in the came, only a "core" selection.
The DM can also create full characters to use to challenge the players, as Humanoid opponents.
But there needs to be NPCs with monster statblocks anyway, such as for hirelings in the economy sections. These statblocks can also represent player friends and relations.
Rules for the DM to create NPC Humanoid statblocks can also serve as opponents against the player characters.
In this way, with only the Players Handbook, the DM can use this selection of Humanoids and Beasts to run a satisfying game, pitting mainly Humanoid opponents, such as Thieves and hostile Mages, against the player characters.