And as you say, it gets even worse when you give some Al-Toril organizations a space presence - notably the Red Wizards of Thay, and the empires of Shou Lung and Wa. If they have access to spelljamming tech and their competitors don't, why haven't they just won?
If you look at the map of Shou Lung then it seems like they have already won in that they already have a vast empire. I am not sure about the absolute advantage that Red Wizards would get from Spelljammers that being able to instantly teleport anywhere in the world does not already give them? Would Spelljammers help them to defeat the Simbul? Most likely just end up in a spectacular crash the same as when the Elves tried to use their Spelljammers against the Devil-Elf Army.
And the issue of trade rears its head as well. Al-Toril is a complete world. There's very little Al-Toril would need to, or even desire to, import from beyond the planet itself.
No one ever trades with a complete world. There are always going to be some bits that need something that they dont want to make - which is why you have trade.
That's why the Astromundi Cluster was a much better Spelljamming setting than Grey-/Realm-/Krynnspace was - it didn't have whole planets. It was all just a bunch of asteroid clusters, which depended on contact with other clusters for survival.
Astromundi might have been alright if you could get out of it but I personally found the whole "locked inside the sphere" idea lame.