I have to wonder if what we are getting here in the SCAG was actually going to be the other stuff that originally was planned to go in the Elemental Evil big player's book that never got announced/cancelled? Perhaps the original intention of the Adventurer's Guide was to come out with EE, but either they determined they wanted more time to playtest a lot of the character options, or that the stuff being written about the Underdark and the dwarven halls for OotA was important enough to hold off the setting guide until OotA was also released?
It might explain why there are no spells in the SCAG, if perhaps the spells that were intended for the EE player's book got split out from the AG and they released them with the EE adventure path book (along with several of the races), and everything else got put on hold to be released now during Rage of Demons so that the Rage of Demons material could be added to the setting material?
It also makes me wonder if perhaps now that we've had three FR adventure paths and a setting/player guide to tie them all together... maybe (just maybe) they're going to move onto a new campaign setting for the next three? Yeah, they'd have to do a little finagling around the Adventurer's League still set around the Moonsea, but if they wanted to move onto a new setting to work on... now's the time to do it (with the campaign guide bringing all the current FR info together.)