I've both played in and ran campaigns that spanned multiple settings. Homebrew -> Greyhawk -> Forgotten Realms -> Planescape, and Dragonlance -> Spelljammer -> Forgotten Realms respectively. Not that most of your post is pretty well correct, but the blanket statement that no one ever has campaigns that span multiple settings isn't...
True. I've done some campaign hopping as well.
In my twentyfive-odd years of gaming I've run three homebrew worlds, the Realms once, Dragonlance twice, Ravenloft five and a half times, and Golarion. But, again, that's over two decades and four-and-a-half editions with five or six different gaming groups.
Really, most people play homebrew. While
a campaign setting is a fun novelty and might be purchased (especially for neat options and variant rules) by their very nature campaign settings are going to appeal to a minority of players. Especially when they're only likely to be purchased by the DM.
The people willing to buy two or three campaign settings is an even smaller minority. The people willing to buy five is probably ridiculously small.
I would quibble that Curse of Strahd was not really a low energy effort. Sure, it reprinted Ravenloft in toto, but Perkins put a lot of effort into the rest. He seemed a little broken after the 1-2 punch of CoS and Sky King's Thunder, so the transition away from 2 APs a year makes good business sense on a couple levels.
Yeah, but to write both, he also took time off work. He spent his vacation times writing both, which isn't always viable and can lead to burnout.
And you can see the burnout in SKT. There's a lot of problematic stuff more time would have fixed and revised.
I think your analysis about PC options is good here, which is why I can see the May release being a Volo's Guide follow-up pretty easily: lots to do in terms of monsters, no overlap with XGtE. Then in November, maybe something with a few player options (such as the UA work from recently).
Maybe.
I think a planar book that's in line with Volo is a good choice. A couple races, a few new monsters, 90% fluff. That's nice and easy.
I highly doubt the fall release is going to be more class content. Twice in a row seems unlikely. Unless it's psionics, but those likely need more time. My money for the fall is still on a book of magic items, possibly paired with the artificer.
But they could do something
completely unexpected just as easily... A book of delves and short adventures. A book of encounter locations. A hacker's guide full of customisation options and variant rules.