I don't mind the Bloodied condition and would have no problem seeing it reincorporated into the game... as it's merely nothing more than one more "timing slot" in which a monster can do a thing. But of course for it to be at all warranted, then it has to go hand-in-hand with more advanced monster action design. Monsters need to have larger blocks with more abilities that function on all the different "timing slots"-- something that
@Nixlord is doing with their Monster Manual Expanded series, MCDM is doing with 'Flee Mortals!' and I'm sure others have as well.
Most monsters should have abilities that occur on most of these "timing slots":
Automatic / Always-on
Action
Bonus Action
Reaction
When successfully hitting enemy
When being successfully hit by enemy
When reaching half hit points (Bloodied)
When reaching zero hit points (On Death)
Off-turn (Legendary)
Of course, the biggest problem with a list like this is that the more monsters you have that have all these options available to them... the more difficult it is for a DM to remember they all exist and to use them when appropriate. I mean I've been DMing 5E now since it was released and I STILL forget to use a Legendary Monsters' Legendary Actions between player turns (just because I'm so not used to it and there are so many other monsters and combat-related activity to deal with already.) Heck... I've begun occasionally just inserting the three Legendary actions into my initiative order just so that I have a reminder as they come up to use them (even if they are supposed to happen organically in reaction to what the players are doing.)
That's the biggest thing with re-adding Bloodied into the game-- it's just one more "timing slot" a DM has to remember as it comes up. And to be honest... when I think about it I'm not sure if the WotC Monster Manual is necessarily the book to use all of these in. I honestly think the MM should probably be and remain the base and least complex monster book... and let folks like
@Nixlord and MCDM make the more complex monsters using "timing slots" like Bloodied. So they're there for those who want them... but less-inclined folks don't need to pick up them up and they can just use the standard MM.