I have to admit I haven't bought much from WotC lately, not because I'm mad at them but they just haven't released anything worth buying.
This hasn't been true pretty much from the start of 5e IME. The release schedule's been pretty empty.
The Wild Beyond The Witchlight - Sounds cool, but is a campaign, so no sale.
Fizban's Treasury of Dragons - Maybe I'm playing the wrong game, but I've never been keen on dragons, and nothing in it seemed exciting. I think that's first sourcebook I've skipped in 5E (rather than a setting book or adventure).
Strixhaven - Is a campaign with a relatively sketched-out setting that has a massive tonal conflict with the much edgier/cool MtG take on the same setting. Again not buying campaigns.
Call of the Netherdeep - Sounds cool, but it's a campaign, so no sale.
Journeys Through The Radiant Citadel - Cute but I need a bunch of short but extremely heavily themed and location-specific adventures like I need a hole in the head.
Spelljammer - A wildly underdetailed, overpriced very straight take on a setting I was only ever moderately keen on. Custom designed to avoid me buying it!
If you aren't buying campaign books and you aren't buying either adventure collections or campaigns then to be blunt what are you buying? Setting books? (Of those I have five - and regret only one).
Putting things into perspective WotC have released their setting books at the rate of
one per year. If you haven't bought one in the past year that's only one fewer sale. They've released adventure books at an average of two per year. Me, I've got all three of those - and the only earlier one I've got is Curse of Strahd. (Dragon Heist, as an adventure with no non-polymorphed dragons and no heist I find offensive).
I did get MotM though, but that's it. Normally I'd have spent a lot more.
On what? You wouldn't have bought the big adventures like SKT or Rise of Tiamat. In a normal year you might have bought one more setting book like van Richtens' Guide or the Eberron Explorer's Guide. But you wouldn't have spent much more than that because you wouldn't have had anything to spend on.
I'm therefore going to suggest the only reason you didn't spend as much as you normally do is that the Spelljammer release is actively a poor product.
Meanwhile WotC are trying to avoid the Diminishing Returns trap. Fundamentally some DMs are insatiable and will buy Forgotten Realms Supplement #23 - but most of them are satiable at some point or other and you essentially can't sell almost indistinguishable stuff to the same people.
They've sold the Realms fans SCAG and a fair amount of Waterdeep stuff - and the further Realms stuff never sold as well. They've sold the Eberron stuff. And for fans of an only slightly different flavour of fantasy they've sold the Critical Role setting. So the last four campaign settings have all been about widening the net for something different in tone to D&D fantasy. You have:
- Mythic Odysseys of Theros - very much Greek Myth and odyssey inspired. Very different from Waterdeep or SCAG
- Van Richten's Guide for running horror games
- Strixhaven for Undergraduate Harry Potter
- Spelljammer for jammin' spells (or rather for space piracy and trading)
- Forthcoming Dragonlance to get to the Giant Good vs Evil that Eberron doesn't do and the Realms has drifted away from if it really does
The point of all these is that they are pretty different from the Forgotten Realms and Eberron - so although the market overlaps it is not the same. Because if you just do the same then people get satiated. Likewise the adventures - I'd run The Wild Beyond the Witchlight with a very different group to Curse of Strahd because it's very different in tone. And Radiant Citadel again is fitting a different niche in when you'd run it and for who.
The only book I'd say that is focusing on the market already covered is Fizban's - and I think it was a trial balloon to see if things like updated versions of the classic I, Tyrant that focuses exclusively on Beholders were worth publishing as another expansion. And the answer I suspect is no.
But fundamentally the current WotC books don't absolutely nail the market covered by the previous WotC books. If they were to then WotC would sell fewer of them than the previous ones. But if they hook one person in with e.g. a Witchlight campaign because wandering through fey realms is what is cool to them then that's one person who might buy an entire collection of books. They're trying to fish in neighbouring waters.