Planescape can exist in fluff as long as whatever they do in the future doesn't contradict the fluff. The Planescape books are fine.
Darksun, Ravenloft, and AlQadim need some rule tweaks to update them, but their fluff is fine too...
Greyhawk is a worthy setting to continue, but they might just want to leave it to the RPGAers....maybe publish a book every once in awhile of all the fan stuff for the setting