Henadic Theologian
Legend
I'd see that if they weren't republishing the core rulebooks three.
Essentials was kinda like Rule Cyclopedia in that it was bundling up the rules in a new package and have new variations on the classes, but everything was published with a new name for clarity's sake. So we have Player's Essentials: Heroes of the Fallen Lands, Player's Essentials: Heroes of the Forgotten Kingdoms, Rules Compendium, Dungeon Master's Kit, and Monster Vault. None of these had the same name and thus occupied the same position as the Player's Handbook, Monster Manual, and Dungeon Master's Guide. But they served as a soft replacement.
2024 releases will be a harder reset, more like 3.5e, if only because it sounds like they're going to be republishing the PHB, MM, and DMG in new forms. But they intend for Xanathar's, Tasha's, and Mordenkainen Presents - at least in their 2022 republished incarnations - to be able to stand in companion with both the 2014 core rules and the 2024 core rules.
That makes me wonder if the Sorcerer subclasses in Xanathars are going to get an update to put them in line with the PHB 5.5e and Tasha Sorcerers, aka bonus spells? Because between those three books that would update all the Sorcerer subclasses (Xanathars- Divine Soul, Shadow Magic, Storm Magic), (Tasha's Aberrant/Psion Soul, Clockwork Soul), and (PHB 5.5e Wild Magic, Dragon Blooded, possible some new ones).
I think that technically the PHB 5e and PHB 5.5e will be compatible, mixing characters in a party that each takes stuff from both books won't break the game, all the mechanics will work, BUT just like having characters that only use books from pre Tasha's at the same table as characters that can use Tasha's, the former will feel extremely gimped by comparison to all the new stuff Tasha characters can.
Really 5.5e started with Tasha's not the 5.5e books which will only bring the core in line with Tasha's.
There is a reason why they had to dump PHB +1 in AL after Tasha's come out, because anyone whose +1 wasn't Tasha's got screwed.
So yeah you'll be able use 5e PHB in a game someone using 5.5e, but it will become quickly clear that you'll want to get using the 5.5e PHB or at least Tasha's as well.