Just making stuff up and saying it's true doesn't make it true. No matter how many politicians favour it. Sad to see that sort of approach on ENworld, frankly.
It would be fair to say, instead, that in the '90s we had a lot of darker games, but that pretty much died out by about 2004. That was 19 years ago. Since then, the majority of games have been increasingly light in tone, to the point where the average D&D game now, particularly to judge from podcasts, is pretty much wall-to-wall wacky hijinks. Critical Role is actually slightly darker than average, but is no real way dark. However, that's what I'm suggesting the target zone for Planescape - Critical Role levels of wacky/dark. If that's "2 spooky 4 u", I think that's a you problem.
If you disagree it would be great for you to provide some examples of how TTRPGs have been "twenty straight years of sadness and anger".
Yeah this is important - the unsettling, dark and grimy side should be there. Not just the "FUN WACKY ADVENTURES LOLZ!". It's not like it needs to be hugely dark - just enough not to be twee or purely "zany".
There is some zany in Planescape, but zany usually has a mirror, too. Like, the Xaositects were played for laughs a lot of times (LOL SO RANDOM), but were also unpredictable hedonistic thugs. Xanxost had a similar vibe - so funny and unpredictable and OH GOD HE'S EATING MY FACE THIS HAS TAKEN A TURN.
I don't want a lot of Acquisitions Inc wink-and-nod kind of stuff, though I bet some of that's inevitable.
Planescape has plenty of fun and whimsy to it as well. The videogame was very noir, but the setting material has a range of tones, with some elements being dark, some whimsical, some weird, and some all three. A lot of it is of a piece with classical faerie tales, actually. In fact, I think a lot of it was too goofy for people when it came out (the slang, etc). I think the goal was to make the familiar become strange.
That's the reason there's so much negativity. Because Spelljammer was dreadful, and that sort of treatment for Planescape would be dreadful.
And WotC explicitly stated, just a few months back, that Planescape would have the same format at Spelljammer.
Now hope has been raised that maybe that isn't true, and maybe it'll have a better format, but mate, let's be clear, it doesn't matter how "open" your mind is - 64 pages for to cover the entire setting and allllllllllll the rules including species, backgrounds, feats, spells, and so on is going to be awful. There's no possible way to make that good. It might 6/10 instead of 2/10, but it's going to be bad.
The first is a banner in the background on the right side of the illustration. It very closely resembles the original Sensates symbol, only there are two hands instead of a hand holding an ear on the bottom (yes, I'm a total Planescape geek and remember this stuff). After Faction War, the Sensates officially disbanded, but many continued to operate the Civic Festhall in Sigil, though others went to Arborea, joined the Mind's Eye, or decided to travel the planes.
The second is tattooed on the forearm of the blue tiefling on the right side of the illustration. The horns are very reminiscent of the skull motif Doomguard symbol, though the "ball of fire" between the horns is new. That's interesting because the Doomguard was basically exiled from Sigil post-Faction War... retreating to their fortresses near the Negative Energy Plane... their factol Alisohn Nilesia was one of the few factols to "survive" the Faction War by virtue of being sold to devils as a slave.
If they've got any sense they will. But these are the people who gave us 4E Planescape which, despite 4E basically being in an entirely different cosmological setting, with fundamentally different planes and so on, still made Faction War canon, and turned Sigil into the Des Moines (or for British people, the Milton Keynes) of the multiverse. So they might not have any sense - but hiring DiTerlizzi suggests at least some working brain cells!