I can't speak on Spelljammer, but Planescape has a metric ton of setting lore. There's no way that a magazine would even scratch the surface of the lore on a single one of the planes.
Yeah, Planescape had at least six boxed sets of setting. It was not niche. Planescape was /all/ setting; it was huge, even discounting the fact that it was technically infinite.
...But does it really need updating? I am really happy with where AD&D2 left Planescape officially. The D&D3 stuff doesn't ruin anything. I just can't believe any further follow-up wouldn't be a disappointment in the shadow of the original material. New factions for Sigil in the aftermath of the Faction War? How is that not going to be a pale imitation?
No offense intended, of course, Shemeska; the planewalker.com folks did a lot of hard work and I respect the effort. I am just content.
Some settings, like the Forgotten Realms and Greyhawk, need an ongoing timeline because history is something that happens in a simulationist fantasy world with kings and wars. But a lineage of Guvner factols for Sigil and changing political borders in the Outlands and major NPC storylines would just crush the life out of Planescape.
A few of the AD&D2 settings were designed to come to a natural close -- Dark Sun and Planescape for certain, and I can't recall if the Grand Conjunction of Ravenloft was supposed to end the setting specifically, but it was catastrophic. I am happy leaving Planescape as it is: a timeless snapshot.
I mean, sure, bring on the tight-focus detail articles in Dragon; maybe even a small sourcebook or two; but I don't want to see WotC waste time and money on a new /version/ of Planescape when the old stuff is still winning.
I feel like Spelljammer had a lot less setting lore.
I don't think anyone has mentioned it here yet, but they've apparently got plans for Spelljammer. Mike Mearls has stated that he feels like it got short shrift as a transitive setting and wants to see it become a fully fledged space opera D&D setting in 5th Edition.
Personally I'll miss the elegance of the Planescape/Spelljammer mashup in the Astral Sea, but it will be interesting to see where that goes.
Go back to the basics by having a generic default campaign setting that isn't fully developed, but reflects more on what 5e is trying to accomplish.
I'm with you on this, but I think the Nerathi swan boat has sailed, regrettably.