1. Fair, but I believe the phlogiston is a big part of Spelljammer, so Mind Flayers at minimum have a perceived presence there. This has only increased in 5E, as the only Spelljamming craft we've seen in this edition have all been nautiloids (there may be one that actually wasn't, but the captain was still a Mind Flayer).
Again, I don't disagree with the fact that Mind Flayers are important, what I think you are still not being clear about is the fact that the Phlogiston is a big part of Spelljammer. It has never been in the campaign that we've run, most of them being inside a few spheres, with the Phlogiston being used exactly like Hyperspace in Star Wars. It's a huge place with no interesting features and almost no chance of meeting someone. What can happen there are intrigues aboard one ship, and the special atmosphere makes things just different, but it's really a small part of the setting.
2. I don't want to retcon Planescape, I think having Phlogiston integrated into the Astral is actually a fairly small change that doesn't really impact the majority of Planescape.
It fundamentally changes the nature of the planes like 4e did.
3. Glad to see we agree on this bit.
I'm not sure what this was about, but we don't have to disagree on everything, you know...
5. I don't really want it to be like Hyperspace. I want it to be more like a ship pursuing another, like on a real ocean, like Pirates of the Caribean. The video is from the Tintin film, where they have a big ship v. ship fight against Red Rackham. Great scene in an ok movie.
Hmm, for me it was a bit too much, but you can certainly do that in the space inside a crystal sphere, which is weird enough, you don't have to run the scene in the phlogistion where it is really like Hyperspace. The space between planets within a sphere is perfect for that and there is no phlogiston there.
6. Well even if it's not (I believe it is, but I digress), I would argue that the Githyanki and Elven Armada should be the biggest forces in Spelljamming routes, whatever form they take. They are arguably the equivalents of the British Trading Company and Spanish Armada, as great large enemies or allies. They make great employers or BBEGs.
You don't need to use the githyanki for that, there are lots of other very dangerous fleets without making the Githyankis one of them. Or you can if you want, I just think it wastes two of the primary githyankis specialities, there movement on the Astral Plane and Silver Swords....
7. Ok, no need to call my thinking "mundane." I personally feel like you have a more esoteric view of the Astral than it has, since it has so many very physical locations there.
Which does not preclude the other thing. What is interesting is the combination of the views, because there is also a difference between being physically in the Astral Plane and projecting oneself Astrally.
Back to the Githyanki, they have their whole city of Tu'Narath there, built on a god's corpse. As gonzo as that is, it's very much a real physical place you can go to, and the center of Githyanki activity.
It's a combination of physical and spiritual, like everything on the Astral Plane, see above. By putting this on a purely physical realm, you are losing half the flavour of it.
To conclude, I think integrating the good ideas of phlogiston would increase the Astral's diversity, make it more important to overall cosmology and to the factions on the MP, and increase the playability of Spelljammer overall.
I think you still have not understood the fact that the Phlogiston is only between Crystal Spheres, and that the Astral Plane is just a transitive plane to the spiritual ones of gods and alignments. Flattening all that into one loses one dimension in the game, which is a lessening of its wealth, for sure.