I mean, even Planescape basically acknowleged that. They described the Astral as the place no one is supposed to go. It's the blank space the rest of the multiverse is supposed to be set in. It's behind the stage. They then tried to build it up a bit again by making it all Lovecraftian and surreal, but essentially, for most settings, you don't need it.

Ethereal is a bit different, but I think you can mostly use it as just a kind of dimension that is slightly off from reality. Ditch the Deep Ethereal and most native ethereal creatures, and just say that Etherealness moves you into a grey, lifeless, incorporeal version of the real world. Half a step in a fourth dimension, you're just slightly out of phase with reality. Like, say, when Frodo puts on the Ring and sees the Nazgul for what they are.