Yes, but we would had an official D&D 3.5 Planescape and an 5ed Planescape, likewise Dark Sun. So one shit, one good and one ok is not a bad relation.
Planescape doesn't work with
dumbed down "streamlined" cosmology of 3e onwards, so a hypothetical 3.5 Planescape would
not be good. Also, the whole reasoning behind 3e was "let's get over with all those countless settings which splinter the auditory and cannibalize our own sales", hence only FR and Eberwhatever. At best, Planescape would be licensed away much like Ravenloft, only making it partly official. And then even Ravenloft 3e wasn't permitted to explicitly refer to other settings, i.e. Bane, Lord Soth, Lathander et al had to be masked out. Imagine Planescape getting the same treatment - being a sum of other settings, it would consist of nothing but obfuscations.
As for 5e, did they even release anything past the core rules and a couple of promo adventures? It's like that whole "D&D Next" was a one-off event.
WoC have reduced the amounts of settings and RPG systems (ADD was outselling D&D). The DnD 3rd move from WoC was a good idea despite some minor things. But because WoC have reduced the settings, they have naturally reduced the cosmology to focus on the core of DnD. If they had intorduced Planescape in 3rd then they would have expanded also the cosmology.
Shannon Appelcline stated that CG were shrinking the industry and that TSRs own products were cannibalising their own sales, but i think that it is a false statement (since i bought many many of the supplements, and if you bought one then you bought more).
The production costs were simply to high and they invested much amounts into bad selling products like Spellfire and Dragon Dice. According to wikipedia: Hearing rumors that the Blumes were trying to sell TSR, Gygax returned from Hollywood and discovered the company was in bad financial shape despite healthy sales. (Gygax: "I was alerted to a problem: Kevin Blume was shopping TSR on the street in New York City. I flew back from the West Coast, and discovered the corporation was in debt to the bank the tune of circa US$1.5 million."
"Gary Gygax: Q & A (Part XII, Page 28)". EN World. 2007-01-21. Retrieved 2009-03-15.) And
David M. Ewalt, in his book
Of Dice and Men, adds that
Spellfire and
Dragon Dice "were both expensive to produce, and neither sold very well".
[26]:174
The other settings were selling quite good, even the Al-Qadim thing. And despite good sales TSR made dept and that can only be due to high running and production costs and bad overreaching investments.
WoC did what they did, to save the rest. FR was the highest selling setting, followed by Dragonlance and Ravenloft for TSR, so it is naturally that FR will be continued and became the core of DnD.
The Eberron (can only remember the name, because i associate it with Oberon) thing seems to be quite popular, but i have never seen and read about it, but it is also a late (2003-2005) addition to DnD.
Edit:
Btw. It just dawned on me as i saw the video for "the legend of drizzt board game" (yes i know that it was made in 2011 and is therefore 4th edition):
But the 4ed was also made for board game modules OR computer games, especially for single player and few party members: Very few powers (per day, per encounter, at-will ) on play cards, high starting HP and healing surges.
Hence the board games: The Legend of Drizzt, Castle Ravenloft, Wrath of Ashardalon, Lords of the Waterdeep, Scoundrels of Skullport expansion.
But it didn't sell and was dislked, therefore they went back and developed the 5th edition as a traditional "streamlined" RPG.
Edit 2nd:
Also it seems that WoC is slowly going digital with DnD Beyond announced last month. So yes they seem to be slowly adapting. So perhaps the digital iron age of RPGs could be on the horizon.
Edit 3rd:
Just for the record what they have released of the DnD Next and 5th.
DnD Next from 2012 - 2014: Dreams of the Red Wizards I and II(Scourge of the Sword Coast, Dead in the They), Sundering Adventure I and II(Murder in Baldur's Gate, Legacy of the Crystal Shard).
DnD 5th edition releases of adventures 2015 - 2017:
Hoard of the Dragon Queen, The Rise of Tiamat, Prince of the Apocalypse, Out of the Abyss, Sword Coast Adventurer's Guild, Curse of Strahd, Storm King's Thunder, Volo's Guide to Monsters, Tales from the Yawning Portal (a collection of some old adventures). Next to be released is "Unearthed Arcana" in my opinion.
There are also some Board Games released that i do not list.
WizKid have relased a Minatures and Board Games for the Adventures and Board Games since 2015.
And btw the core books and translated into german and will be released on 1. April 2017 and 1. Mai 2017.
The starter set (Einsteigerset) will be released on 1. Juni 2017.