You said that folks would not come back for one book if they did not also have the expectation that more would be forthcoming.
More frequent meaning in addition to their approach so far. In 2016, the put out Curse of Strahd and Storm King's Thunder as adventure paths, and Volo's Guide is coming up in a few days. Two adventure paths and then a general release book (not to your definition, but this is kind of the point) in November. Last year we saw the SCAG in November.
The game is using FR as the default setting. It's baked in. You won't see a product that doesn't reference the Realms unless and until they introduce another setting. Therefore, SCAG and Volo's are the general release books that WotC has produced. You just deny that they fit that category because of your personal preference.
Hence, your desires are more specific and more demanding than the average customer. You want them to redefine what they consider a general release, and you want them to go from 3 books a year to 4. You say "it's only one book" but would you say "it's only an increase in production of 25%"? Asking anyone to do 25% more is significant.
The Sword Coast doesn't exist. It can be anything I want it to be, or anything you want it to be. There is plenty of material to be found in that book that can be used in a general way, for a game in just about any setting. Yes, the material assumes a specific setting by default, but so what? Almost all RPG books have some default setting, or implied setting.
I don't know if those numbers are accurate or are even meant to be accurate. But what they put out in the prior edition has no bearing on what they are doing now, except that it has influenced your expectation. Their business model has clearly changed. You don't like that, and that is of course fine.
What I find odd is that you see the idea of you adjusting your expectations on what to expect from WotC based on current evidence is the less sensible option when compared to WotC altering their publishing schedule to try and earn your business.
I can still find useful material in a setting specific general release book like SCAG and Volo's - and I freakin' HATE the Forgotten Realms (at least from 3E - present realms). It isn't hard to strip out setting specific stuff and adapt it to another setting. That kind of thing is happening for years. I get it, people are strapped for time, I am too. I work full time, I'm in graduate school, I have a family and responsibilities and I would love for my specific ideas of the ideal TRPG crunch book to be created by someone else and made available for purchase but I also realize that what I want and what I get are two different things. If I think the book has some usable information in it after perusing it at my FLGS, I'll throw them my support and buy it. If it doesn't have anything useful, then I vote with my wallet and don't give the publisher my support.
Having this debate is good and it definitely demonstrates that there are some very ingrained and rigid ideals among the TTRPG community (as there always have been - think back to the edition wars on the late 2000s - many goblins died to bring us Pathfinder during the bleak 4E years). I will say that in my 41 years of life and 31 years of TTRPG gaming, I've learned that most of the time if I want something very specific, I have to create it myself, usually from pieces parts from other resources. It isn't ideal but sometimes the only way to get what I want.