Guys. What it’s called really doesn’t matter. They’re revising the rules. The changes to spellcasting monsters in the new Mordenkainen book prove that they’re not afraid to make some mechanical changes to existing rules, but the statement of full backwards compatibility means they aren’t planning to change anything so much that you won’t be able to comfortably use old material alongside new. Call it 5.5e, call it 5Essentials, call it 5e revised, call it 50AE, call or Advanced 5e (ok, maybe not that one), it doesn’t really matter. Whatever you call it, it’s going to be a revision significant enough to merit new core books, but not significant enough that material from the first 10 years of 5e won’t be usable with it.