See, this is leading me once again to be slightly confused. I've heard people say for years that they want Spelljammer so they can take characters from FR to Krynn or something similar. That was the purpose of Spelljammer, in the ways it was explained to me.
But it is also the purpose of Planescape, and I have often seen the two settings put at odds with each other, as in you either had a multiverse of the PLanescape model, or the Spelljammer Model. This thread it quite literally the first time I've heard it said that they don't cover the some conceptual space.
I understand now. You are working under a misconception.
The purpose of Spelljammer is to have space adventures in D&D. I bought Spelljammer the day it came out. I read the old
Spelljammer comic book (yes there was one), and I have DM-ed using it. It's a blast.
The person who explained Spelljammer to you took one of the secondary characteristics of Spelljammer and elevated it to "the purpose of Spelljammer." The purpose of Spelljammer is to have fun by having space adventures using D&D. That would be about it.
There was always the possibility to go from Oerth to Abeir-toril using magic or traveling through the planes well before Planescape was a glimmer in Zeb Cook's eye. I would also contest that the "purpose" of Planescape is to "take characters from FR to Krynn or something similar," as you suggest in your second paragraph. The purpose of Planescape is to have a D&D
campaign setting focused on multi-planar travel and multi-planar philosophies and politics. The travel between different Material Plane worlds like Abeir-toril, Oerth, or Krynn is a
consequence of that.
Concerning, "This thread it quite literally the first time I've heard it said that they don't cover the some conceptual space." Well, at the risk of
shoulding all over you, perhaps you should try to look up the old Spelljammer & Planescape material? It's fun.
You have been responsible for a lot of conversations here and have put a lot of energy into making arguments and clarifications all stemming from an apparent misconception.
Once again, a character can travel from Krynn to Oerth with Spelljammer and also through multi-planar travel, but that is not the
singular purpose for why Spelljammer or Planescape exist by any means. Gary Gygax in the
Dungeon Master's Guide makes it clear that having characters visit different worlds can be a fun part of the game, to such an extent that he provides mechanics to convert D&D characters into Boot Hill characters, and that was 1979.
We were moving characters from world to world well before Planescape, well before Spelljammer, well before
The Manual of the Planes.