I prefer what the current MM has. I prefer all demons and devils grouped together so that I can flip past all those things nice and quick.
The only reason I'm okay with every undead or humanoid being its own header rather than all together in a group is that those monsters tend to be ones that I think of and identify as themselves individually, rather than part of a group. If I'm making an encounter, I'll say "This would be a good skeleton encounter" or "There's a kobold tribe here". Whereas if I'm making an encounter using Demons or Dragons or Giants or Elementals... I always think something like "I want them to face some giants here" and then look in the book to decide what giants I'm going to use. Or "They are going to meet a Genie!" and then decide what genie after I look in the book.
To me... I equate the individual demon types or individual dinosaur types or dragon types, giant types, etc. to the different types of gnoll statblock-- the base Gnoll, the Gnoll Pack Lord, and Gnoll Fang of Yeenoghu. They are all just variations on the main monster, which I tend to consider to be demon, devil, genie, elemental, dinosaur, dragon, gith, mephit etc. and thus I want those variations all in the same place.