Why would you think Planescape is dead?So is Planescape like, dead dead?
Those Spelljammers have to have destinations in the Astral after all, and Mount Celestia and the Nine Hells are as good a place as any for folks looking for adventure. And Sigil is going to show up again because Sigil is just a cool idea.
(Now is the Great Wheel as be-all end-all of the Outer Planes an idea that is dead in this edition? Time will tell. I suspect yes - that they'll adopt a more expansive approach to the Outer Planes and the Great Wheel will only be part of it, but we'll see).