Hussar
Legend
No, 2e already forced every world in the same multiverse. 4e simply made the misstake of forcing them into a different cosmos. 5e is going back to how things have been in D&D for the longest time. Why would you lose / not use them? Just take the stats and ignore the fluff. I can take a balors stats and say in my world these are the stats of the Rabbit of Caerbannog and not give a **** about what fluff these stats originally came attached to
Except that Balor's, for example, don't exist in a vacuum. I play with players who have all played for a decade or more. All of them have DM'd. All of them are fairly well versed in D&D lore. If I drop a Balor, that looks like a Balor, but, isn't really a Balor, they're always going to have questions on the mind. If I feature an adventure with a Balor side by side with three Ice Devils, it can get problematic, if I have players who care about D&D lore.
So, largely, most of the really strongly Planescape tied monsters, I simply ignore. Makes my life easier.