That's not how it works in 5e. They are separate systems. I know that they exist in the same edition, I want both and was never advocating for one over the other. Both systems are good, and I want them in 5e.
They could change it, but that would require invalidating some already published 5e information on how the cosmology works.
Yeah, I was never advocating for Planescape instead of Spelljammer or vice versa. I have both a Descent into Avernus and Spelljammer game in the same universe. I'm just saying, if/when they bring either system into 5e, they're going to stick with keeping them separate. I am well aware that Mordenkainen's has Gith and Giff and demons/devils. It's a key part of my world. Volo's even has neogi and a small amount of nautiloid information.
Except the fact that they'll have to distinguish between M:tG worlds and original D&D worlds. I would prefer to have Planeswalking only work for Ravnica, Theros, Innistrad, Ixalan, and the other M:tG settings. These settings would not have crystal spheres. Greyhawk, Krynn, Toril, Dark Sun, Wildemount, these would have crystal spheres and be accessible through doors in Sigil and Spelljamming vessels.