So I agree with most of this. What I disagree with at this stage is whether that requires a FRCS, particularly with the amount of material that has already been released.
The jump was really a matter of a couple of years, from 4e to 5e. It has already been stated (in SCAG) that the effects of the Spellplague (mostly the "returned Abeir lands were gone, the large rifts that had formed in various places fixed, the Sea of Fallen Stars was back to its normal banks, and many of the Gods had returned) are largely reversed. Shade is gone (which was actually a 3e thing), and Anauroch is a desert again.
What this means to the world depends a lot on your campaign. In my FR all traces of returned Abeir (including the dragonborn) are gone, but the shadovar will remain in a very small scale since the fall of Shade.
What they haven't detailed is much in terms of the rulers, but presumably most of that remains as detailed in the 4e FRCS and NCS unless new material says otherwise, such as Lord Neverember being removed from being a Lord of Waterdeep.
I suspect there is a gap between people who acknowledge the 4e material and the 3e and earlier material.
I do agree that we need a book like the SCAG for Cormyr/Heartlands/Sembia/Moonsea, and probably one for the lands south of Baldur's Gate. An overview of the political state and such would be fine with me, but a FRCS that repeats the material released in the APs and SCAG would be a mistake, as would releasing a "FRCS" without it. So a series of regional books makes the most sense (and appears to be what they're doing, at least to me). And if you're interested in running your Realms outside of the Sword Coast, you probably don't need the APs set in the Sword Coast. You'll probably still want the SCAG though. I see the material in the APs as extremely helpful for DMs running the Sword Coast and the North, but probably unnecessary for those running a campaign elsewhere.
Eventually one that covers the east (Thay Mulhorand, etc.). Beyond that, everything south of the Sea of Fallen Stars, east of Calimshan, and west of the Old Empires. Those seem to be the major sections that have received regular coverage over the years.
In many ways, I think that an AP set in a specific region will probably highlight the differences of actually being in those regions better than a campaign guide that just covers the region in much the same way CoS (or even the old I3-5 adventures, particularly the expanded super adventure release).
More than anything, though, I think that the material you are looking for will be released, eventually. But I doubt it will be in the form of a FRCS. It may not be what you'd like to see, but I think it's pretty clear that's the direction they're taking. And it's unlikely to change if sales continue as well as they have been.