Much as I like Planescape and Dark Sun, their popularity outside of guys in their 40s-50s who still use the word "module" even though WotC stopped using it 20 years ago is minimal - and even in their heyday, these settings were not very successful financially. The fact that this board overwhelmingly consists of that demographic could lead one to believe there is a demand for those settings, but WotC's market research doesn't bear that out. Each has a Gen X cult following but not a growing fanbase.
Exandria is the money setting right now. Nothing they release in 2020 will outsell the Wildemount book. If they don't release more Exandria stuff, including a hardcover adventure, they will have dropped the ball. And I'm not saying that because I particularly want Exandria stuff; it's just that it's very clearly the best business move they could make right now. WAY ahead of Planescape, Dark Sun, or Spelljammer.
Kara-Tur or any iteration of "Oriental Adventures" is a PR disaster waiting to happen for them unless they hire a team of Asian creators to make it. Otherwise I'm sure they'll have the common sense to avoid it.
Valid points, all, but I'll throw in few points.
Yes, PS and DS are beloved by us Gen Xers, but they are
really beloved. But further: the question is how they'd translate to the contemporary context. No one knows for sure, but I personally think they'd be quite popular among the young 'uns.
Furthermore, the reason they don't have a "growing fan-base"
could be because there is no new material. WotC doesn't have to go whole hog; a Manual of the Planes is a near inevitably for reasons I stated, and they can test out Sigil/Planescape within its pages, and then decide on whether to do more specific treatment. So maybe I should push back my
Shemeska's Guide by a year or two, and go with the idea of a dedicated Sigil chapter in MotP (like 4E), with possible greater coverage later on.
DS also fills a need, or a lack in the current lineup: a heavily thematic sword & sorcery style setting. It may not receive a line of follow-up books, but I see it getting the Eberron treatment: a core book (with a possible player's guide including psionics), and then opened up to DM's Guild.
Kara-Tur doesn't have to be a PR disaster. I'm not sure how D&D is doing in Japan, South Korea, and China, but if there is a market, it could be designed there as the first "international D&D book."