Warning: I'm going to use caps for emphasis.
But the issue with a 5E FR campaign setting book as exhaustive as the 3E book is two-fold:
1) How many man-hours of writing, developing, illustration, and mapmaking would it take to produce it, and how much would it cost to make? Is the current staff of D&D R&D capable of producing a product as extensive as that?
2) Does the fact that it would be a setting that takes place in probably around 1489 DR (ten years after the 4E book, and still 100+ years after the 3E book because they wouldn't be editing out the Spellplague and timejump from the history) make this book less of a desired product for a large swathe of the Realms fandom, *because* it doesn't eliminate the Spellplague and 100 year jump from existence? Do Realms fans who normally would buy a setting book refuse to do so in this case specifically because it maintains a hated part of Realms history for a lot of them?
I completely understand many people's desires for a new setting book. I do. But let's be real here-- a good percentage of the Realms populace won't embrace and will not buy a product that assumes everything that happened in the 4E Realms still happened (even if a lot of unpopular stuff gets "written out" via magic and the reforming of the Tablets of Fate). Which means a percentage of the people they might expect to buy said product actually won't. So if you can't count on the sales of your gaming populace... why would you even want to attempt to spend the money to produce it when there might be a good chance you'd lose money on the endeavor?
Here's the thing, though. The 3e book came out in 2001. Fourteen years ago. That means, with the exception of myself, EVERYONE in my gaming group was 12 or younger! And they're GRADUATE STUDENTS now!! None of them have even seen the FR grey box. One might have seen the 3e FRCS. I don't think any of them played 4e much. There's at least two generations of gamers that don't have what I would consider a proper campaign setting. That's a market.
We're butthurt about timeline changes n' stuff, but we're at least a generation out of date. Probably two. I hate it when people say this to me, but...we're not the market.
Can WotC even write something like this? Hell yeah. Some material can be reused from earlier works, and...this is what they do. Can two people write five chapters each? I would hope so. Three people could each write three chapters, and a fourth write one. It's doable. (I'm just picking 10 chapters for kicks and giggles, btw.)
Is there a market for it? I dunno. There was a market for that Forgotten Realms hardbound last year that was, insofar as I could tell, a collection of essays by Ed Greenwood. I have a hard time believe that would sell more than a full-on CS. The people who are going to be aggrieved and vocal about it are either going to buy it to complain, or buy it because they're completists, or not buy it. That's two win scenarios and one neutral (you're not really losing a sale there). Between the completists, the new generation, and the people who are just curious/interested, I don't think the gripe-squad is nearly as large as the internet likes to pretend.