I reject the proposition that an introduction should necessarily be quick and linear.
As someone who has introduced a LOT of new players, I do not reject that proposition. Linear means they know where to go next instead staring blankly at me and having a meandering discussion for 30 minutes. Quick is just good.
Heck, I was introducing new players, and in the first session gave them a simple quest. Find a Priest's amulet. He lost it, he wanted them to find it. We got four sessions in, lost enough players we restarted with a different story, and never got to that amulet, which was supposed to be a quick level 1 encounter.
It should have something meaty in it. If not more setting, it should at a minimum have some way to generate places for the setting. I'm disappointed that there's basically no advice for creating planets, systems, or the like in it, for instance.
I agree, but does that make it a bad setting book? I'm not convinced. We know what the types of places we want are, we just want a tool to randomize and mix them. That tells me the setting has done an excellent job of expressing itself.
I'm not saying that you're wrong to say it's good enough for you. I'm just saying that I find it very, very disappointing, and that, moreover, the things I find disappointing are consistent in recent releases- everything after Van Richten's Guide to Ravenloft, which, in contrast, I find fantastic and full of great advice and techniques for running horror games of all kinds. VRGtR was meaty; regardless of how closely it hewed to previous RL material, it provided what you need to run horror. It was like a great hunk of meat dripping with juice. In contrast, the 5e SJ book is like the bones left after you eat a steak: you can get a few scraps from it, but if you're hungry for D&D in space, you basically need to cook a new meal.
What advice do you think is necessary to running a "ships in space" game? This isn't a facetious question. To me, unlike Horror adventures which are very unlike traditional DnD, Spelljammer adventures are classical DnD. Explore the Unknown, delve into ruins, and fight monsters. I don't think they needed to explain how to run these adventures, because they are just classic adventures set in a different setting.