The 3e FRCS is in some ways a distant prototype for the current 5e setting books. It's very similar in many ways, although it had a very minimal bestiary compared to the current books (which is likely due to the near-simultaneous publication of Monsters of Faerun, which basically functioned as the bestiary for the FRCS). It even had a couple of mini-adventures! Obviously, a 5e setting using the current format wouldn't be all that far different than the 3e version's style. And honestly, I would buy a 5e FRCS in the style of Ravnica or Eberron in a heartbeat.
The existence of SCAG and some of the adventures might cause some other issues though. For example, for all the flack SCAG gets, the deities and races parts were actually quite good, and that info would need to be repeated in any 5e FRCS - but that might cause some complaints of double dipping. And while Ravnica and Eberron both have a "city of adventure" section, 5e adventures have already done the same with in-depth Waterdeep and Baldur's Gate descriptions, so, it's either repeat that info (see above for possible complaints), choose another city (but which one would have as much clout as those two? Neverwinter, maybe?), or skip that section altogether (which, actually, might be ok as Wildemount has done so, and it would free up precious pages for other stuff).