A lot of people talk about the quality of WotC products.
I really don't feel that it wasn't until Spelljammer that quality finally went off a cliff. And that's because I think that Radiant CItadel marked an increase in quality, one that left me hopeful, as did the new monster designs in Mordenkainen's Monsters of the Multiverse (or w/e its called). Likewise, the Dragonlance book was a pretty decent adventure! Its a railroad, but it is intended to be a railroad, and the adventure has fun epic moments, is greater for beginners and veterans, and presents Dragonlance is a fun, one-off way.
Ultimately, though, Spelljammer denotes something that I think is a little worse, which is, WotC really doesn't like developing new rules. They hate making subsystems, and they hate making additions to their current systems. They do so a little bit, but they are so restrained that you end up with maybe a subclass, some feats at level 1 (this takes no designer skill whatsoever to come up with), and so on.
I don't think the designers of 5E are unskilled by any means. I like 5E's core. I like some of their additional mechanics, especially those in Van Richten's (the survivor and sanity mechanics are favorites of mine). But, the company's design philosophy is to keep to a certain status quo, and only over 10 years begin changing that status quo as they lead up to a new edition. I think this is a little too conservative, and that books that are meant to explore mechanics end up being lesser quality because of it.
Beyond Xanathar's, mechanical additions from WotC have been controversial. The ship fighting rules in Saltmarsh are mediocre and fail to make it into Spelljammer, who instead uses even worse, more controversial rules. Most of the rules in the DMG that are variants never get added to other systems; instead, they get phased out and replaced with more streamlined versions (not all bad, not all good). Many non-controversial options also never get talked about. The evolving magic items in Fizban's, for example, or the aforementioned Survivor rules. Because they don't get talked about, they get forgotten in online discourse, and so the conversation remains that quality is dropping.
But is it? I don't think so. I just think the conservative philosophy of WotC's design process means their end-of-cycle books are weaker. Instead, WotC should have put more effort into making deeper-yet-streamlined mechanics (like evolving items in dragon hoards) instead of phoning it in (like backgrounds in Strixhaven).