Henadic Theologian
Legend
My personal suspicion is that if crystal spheres are maintained the mechanics of them will be changed. I suspect that not every setting will be in a crystal sphere because it's way too limiting. Just like every setting can have its own cosmology, different settings can have their own fantasy physics as well. Mystara, for example, was never in a crystal sphere - and is by canon in a galaxy where space travelers with advanced technology have visited it in the past. While that can be justified with crystal spheres, it shouldn't have to be. So some might be floating in crystal spheres, some might be in a universe much like ours, and some might have some other structure entirely.
(The crystal sphere cosmology was a hack to allow Krynn into the Spelljammer model. It was the answer to "how can Krynn's constellations disappear when the gods are walking on Krynn". I remember an interview with Jeff Grub IIRC where he talked about the origins of the crystal spheres and that was his big rationale for them. And it was a way to make it so that the fantasy worlds weren't tied to each other in the same galaxy or something like that - you don't have to worry about explaining how they relate to each other in space if they're all just floating in crystal balls in some kind of explosive ether substance. Personally I would have preferred portals between alternate Prime Material Planes as the answer to that question but I wasn't on the design team.)
You can have an entire galaxy or even a galaxtic cluster in a Crystal Sphere, hell Eberron shoved like over a dozens planes into theirs.