I'm personally a little torn on this; I think the SCAG does it's job, being a guide to the Sword Coast. It certainly isn't a guide to all of the Forgotten Realms, and was never intended to be. And I certainly wouldn't be upset if FR got another setting book detailing areas beyond the Sword Coast, with updated art and less-stereotypical lore.
However, I struggle to believe that Forgotten Realms deserves a second book. People are quick to point out that it appears like the most popular setting, but I believe that has more to do with it being the most known setting (it is the default in 5e, and has been in place in the most editions of D&D).
And the other setting books that have/will be released are not exactly guides to their entire planet's of material either. Ravnica and Theros are, but are both incredibly monolithic in their content (Ravnica is one city, and Theros is dominated by the same culture and pantheon). Eberron and Wildemount are arguably just as limited in scope as the SCAG is, as Eberron is a guide to Khorvaire (there is precious little material about anywhere else), and Wildemount even admits in its name that it is a guide to its one continent, not all of Exandria.
Plus, Forgotten Realms has benefited (or been hurt by, depending on your POV) from being 5e's default setting. There have been a total of ELEVEN adventures books or boxed sets that are in Forgotten Realms, and their release has given even deeper content for the Underdark, Chult, Waterdeep, Baldur's Gate, and Undermountain.
I fully expect that this pattern, of the annual adventure book containing content for FR, will continue.
I'll ask FR fans this; would you be willing to trade one more setting book, for not getting annual adventure books? I doubt it, but I'm perhaps mistaken.
It deserves it because the Forgotten Realms has consistently delivered more then any other setting, even when 4e divided the Forgotten Realms fan base. There are a huge amount of FR fans who want a book like this.