To be honest, it's less making a case for or against as much as predicting the inevitable. The logical endpoint is that the concept of monsters will become irrelevant.
During the Star Wars saga line, WotC put out a "monster" book. It was mostly filled with various alien species that existed in canon, separated into human-like species (the vast majority of them playable as PCs) and beasts like rancors or reeks that were either animal-like or way too powerful to be a PC (or both).
I predict the next "Monster book" that replaces the Monster Manual in 6e looks a lot more like that. A lot of formerly "monsters" will be redesigned to be PC friendly both in terms of lore and mechanics (imagine for example the minotaur, centaur and satyr from Theros becoming the MM default stats). The only things not done like that are beings too powerful (dragons, fiends) or not sentient (golems, beasts). The wall between monster and PC will more or less be gone.
It will be a very different take. Not completely sold on if it will be bad, but it will be different and that will make 4e-sized waves.
What happens next is anyone's guess but based on how PF2e is going, I doubt Paizo will be the safe harbor it was in 2009 for those seeking to avoid these changes. It will be a momentum shift that no one is going to be immune to.