Of course they could always do a new one, like Nerath.
Tragically, Nerath is dead. It basically existed as an amalgamation of Greyhawk and Mystara, and was a great idea for an edition that was not planning on supporting either of those settings but needed a generic home location that called up some classic D&D oldies-but-goodies. In an edition that in the best case scenario is going to provide new Greyhawk material, or in the worst case scenario is going to roll Greyhawk's notable IP into the Forgotten Realms, Nerath has lost its place and purpose in either case.
I think that for those wanting to know the state of the Realms post-Sundering, sure, a CS book is necessary.
The grim reality is that this book is the 3rd Edition FRCS. Speaking as someone who actually liked the Spellplague, the Forgotten Realms have undergone the tabletop equivalent of a server rollback. Expect anything that the Sundering actually updated to be quietly brushed under the carpet. I give Salvatore two or three novels before he just straight up starts calling Drizzt's companions by their old names again -- if he hasn't already.
From the reviews (as I do not have a copy yet), one of the striking details of Princes of the Apocalypse is that it is more sandboxy than Hoard/Rise was, and each "adventure area" is covered in a way that allows it to be used outside of the scope of the campaign at large. Therefore, it seems like they purposely made it easier to just tear out and use sections in your home campaign.
Oh, no, this is absolutely true. PotA is supremely modular. But that's actually my problem. I'm smart enough to port anything to my homebrew world, should I choose to do so -- what I wanted was for the book to tell me more about interesting parts of the Realms where the adventure takes place, or more about the threat of Elemental Evil to D&D as a whole, or both, and I don't feel like it effectively does either.
The adventure is set in back country inland from the Sword Coast, and yes, it's back country that could just as easily fit into a homebrew world, Krynn, or Oerth. But who cares? It's back country, of course it is generic. It's /great/ for a homebrew game, but it tells me nothing of value about Toril.
Similarly, the villains of the adventure are cut from whole cloth. They've never appeared before. That's a good thing, but the problem is that they are citizens of Faerun in support of a threat that is entirely new to Faerun, so their backstory comes off as ungrounded. And that's a bad thing.
I'm not learning anything /new/ about Elemental Evil because the portions of the adventure devoted to the threat spend all their time developing new ties for Elemental Evil to exploit in Faerun. This does not interest me, because these ties were unnecessary to begin with. Faerun has plenty of well-grounded threats of its own; it didn't need another one.
If I'm buying a sourcebook it's because I want to learn more about its setting, but my interest in 10,000-foot-view geopolitical information is limited. It is useful to a point, but I learned a long time ago that campaign setting books are not really smart buys for me, because they don't tell me a whole lot about what's going on on the ground, which is what I really want.
This AP strategy could be a great way for dungeon masters looking for modules and dungeon masters looking for campaign settings to meet in the middle, but in order for that to happen the format has to change and we need to see more granular setting-specific content, not setting content that is so bland it could fit anywhere. The latter option only serves the module-seeking dungeon masters. Sure, it's Toril, and sure, it's detailed, but it doesn't matter.
I hope that my position is clear; my feelings are pretty complex and I think I jumped between streams of consciousness a bit in there.
