Great Old One
Well for one thing, you weren't arguing in good faith by calling me names and by arguing against points I wasn't making... for example, I said I don't like that Beholders/Flayers having such a significant role on a region that is supposedly part of the Material Plane. You countered by saying that they aren't random encounters, which is not what I said at all.
Then please explain what you mean, because I don't understand why you specifically assign these creatures to the phlogiston why there is no justification for it. They are not encountered natively in there, anymore than elsewhere on the prime, for example, and if you encounter them there it will be in spelljamming ships like any other race spelljamming through the multiverse.
I think the most important pieces of the phlogiston is that it is relatively safe routes of travel from sphere to sphere. I think you can take that same concept, there being "safer" routes of travel, and put it in the Astral Sea.
Remember that the Astral sea is only a 4e concept. All other editions including Planescape call it the Astral Plane for a reason.
It is after all called the "sea" makes more sense to sail there eh?
See above, 4e did a lot of things right not only in terms of rules but also by inventing the Feywild and the Shadowfell, but that "compacting" of the multiverse and the radical treatment that they did to the planes of existence was for a lot of people a catastrophy, in particular for people like me who loved Planescape - and there are more fans of that setting than of Spelljammer and probably 4e.
And you can remove the messy bits of why phlogiston being flammable, an awful mechanic as it kind of ruins a lot of the fun of Spelljammer v. Spelljammer combat (no fireballs! No cannons!)
First, combat in the Phlogiston is not the norm, it's only between crystal spheres, but inside the spherew, where actually most of the fighting takes place because this is where you can hold territory (as you pointed out, the phlogiston is mostly long travel routes, a bit like the open sea where your chances of an encounter are very minimal), you have zero problem with fireballs and cannons.
Second, it also makes for variety, and forces people to cater for different environments, which is always fun when you need to adapt, and that with very little cost in terms of game design.
And like I said before, doesn't make a ton of sense that the Githyanki have a huge presence on the Astral Sea and are prolific invaders of other realms, but are not the dominant force on the Phlogiston.
There is no dominant force in the phlogiston, neither do they need to be a dominant force inside the spheres, as they can raid them at will without coming through the intervening space, which is even more scary.
But like I said before, I'm clearly not going to convince you and your not going to convince me, so I'm tiring of this talk.
The problem is that you dropped a post which is full of preconceptions and misinformation, so you should at least be prepared to discuss these assumptions of yours...