Jester David
Hero
I don't think they planned on three back-to-back settings either. It was probably planned to be Eberron, reprint adventure, Theros, new adventure. Until chatting with Matthew Mercer and them deciding to work together.You are probably mostly right on here, but I would have bet against three campaign settings in 9 months (4 within a year, including Acquisitions Incorporated). I don't see much. Reason to think the Setting market is saturated, since they've turned the books into XGtE plus Volo's Guide genre booster packs. They can keep popping those suckers out a couple a year.
I think the setting market is saturated because most people run homebrew and you only need one or two setting books for the entire life of the edition.
You don't run more than one campaign in a year (if not two), so there's already enough setting lore do do people until 2023. Assuming no repeats.
I have four campaign books for 5e on my shelf (Midgard, Tal'dorei, Eberron, and Ravnica). After I finished my review, I don't think I opened any ever again. Wildemount and Theros would just join them gathering dust.
While I want to run an Eberron game sometime, that's like #4 on the list of planned campaigns. Years away.
I don't know what setting they could even do at this point.
Dragonlance's big story was done in Tyranny of Dragons. Ravenloft had a cameo in Curse of Strahd, and they're unlikely to go back. Greyhawk is divisive and full of some pretty old, racist tropes (and, arguably, doesn't need updating as you can just get PDFs of the books from the '80s and use those: no mechanical updates are required). Ravnica filled Planescape's niche, making Sigil redundant.
Really, the only setting left is Dark Sun. But they need psionics first, which is likely this fall's book. Which could very well work in 2021 as the sole setting of that year. Especially if they include the psion as a new class. Which would encourage non-DS players to buy the game. (Which might be the reason that class seems to be absent from the fall 2020 book.)
They might do what they did with Eberron and have a paid playtest product on the Guild this summer with the final book out next fall. But I think some people feel burned at paying 50% cover price for Early Access and still being required to pay full price for the final book. (I feel salty about that personally.)
If they released a Dark Sun book on the Guild, I think a lot of people would skip it and just wait for the final release.