That said, I agree with the above quote; it's not any grand conspiracy as much as economic sense. Let's say WotC wanted to do a proper GH ToH sequel in 5e. They'd either have to release a GH book to introduce the names, concepts, Gods, etc or waste a significant amount of space in the module to introduce these concepts. Then you'd have to coordinate with the AL to figure out how that season would work (no small task to either start a new living world or port PC's from FR to GH). Further, it would limit options such as removing SCAG options from play. All that for one module.
I'm cognizant of these concerns, but not really sympathetic. One of the design goals of D&D should be to promote diversity of settings. That doesn't mean we have to go back to the 2E money pit of a bazillion published settings. It does mean that, assuming WotC does have a goal of "a single unified experience", they have goals directly at odds with what I see as something that's inherent in the core of D&D.
I also don't think it's nearly as hard as you make it out. The Greyhawk gods are already in the back of the PHB. The original ToEE and AtG didn't need a deep dive into Greyhawk -- they had the maps they needed. All the pre-SCAG adventures worked just fine in the Realms w/o having a separate source book for the setting. I don't actually have SCAG, but my understanding is that many of the options aren't actually Realms-specific and that those that are (Purple Dragon Knight) actually have conversion options for other settings.
Of course, I'm also very strongly of the opinion that each published setting should have a few things that actually are unique to it and distinguish it from the other settings: Warforged and Dragonmarks for Eberron, Muls and Defilers for Dark Sun, etc. Other than a couple of sub-classes, Greyhawk and the Realms are probably the exception. So, implicitly invalidating a very small amount of the SCAG is more a feature than a bug, IMO.
I'm not an AL participant, so I'll totally grant that I don't understand the implications of switching worlds. I also don't have any idea of how much business the AL drives to WotC. Again, I'm not overly sympathetic. If AL is that big of a money machine that it warrants the lion's share of the attention, so be it. Just start putting an "AL" logo on all the Realms stuff so that it's clear that's a subset of D&D. And, then, produce some non-AL content for the rest of us. Also, stop borrowing names from the Realms for books that aren't for the Realms -- especially stupid ones like "Xanathar". At the least, borrow names from multiple settings, if the direction is to use proper names for the books.
The closest analogy is Ravenloft, and there is a very good reason why they went back to "weekend in hell" rather than "campaign setting" Ravenloft. I'd wager a planar/Planescape module will follow a similar path. The release schedule doesn't really have a way to support multiple settings at this time.
Planescape is actually the one setting that
could work as the default backdrop for the game. I've never particularly cared for a lot of the Planescape-isms, but they've worked their way into the game and it's supposed to be what unifies everything. If you want your AL group to follow the various published adventures, then just be aware they're passing through Sigil on their way between seasons. It also helps to permit the Purple Dragon Knight and the House Cannith Artificer in the same group (even though Eberron doesn't work particularly well with the Great Wheel).