Quite. It might not be a "zero sum game", but it's clearly they believe there are limits before they're competing with themselves. As you say, Wildemount appears to have been developed externally, so it's hard to really count it as WotC beyond that decided to publish it, and presumably make more money for both them and CR, than if it was external.
I'm sympathetic to this position, but when you came to the FRCS, was the FR new to you? That's a non-trick question.
I ask because, had I come to it like that, I think it directly would have, as Taladas did. I can't honestly say if it did, because I came to the FR in 1989. What did directly inspire me to run the FR was FRA. And that kind of supports your contention here, because that was more detailed than the FRCS. But it was also a lot more detailed than SCAG, which, even in the Sword Coast areas, doesn't really feel much more in-depth than the FRCS (it may be less, in some ways). The weird thing about FRA was, too, that it was almost SCAG, in the sense of what it focused on, but it such a better job of providing engaging detail that it's hard to compare.
So yeah, really, ideal world, a bunch of stuff like Silver Marches would be great. I think you could make it sell really well just by putting in good mechanical stuff (subclasses, races/subraces, backgrounds, feats, etc. etc.). Maybe we don't need another FRCS. But I'd love some more stuff like you describe, or like a lot of 2E and even 1E products, which do provide more detail. Or just someone giving poor Ed a bloody budget. I genuinely love what's going on with Border Kingdoms but the lack of maps, illustrations and proper layout just make me feel really "FR on a budget", which is sad when I look behind me and see tons and tons of 2E and 3E FR stuff that wasn't on a budget. And yes "go use that!" one can say, it's fair. But I don't really want to be hauling old-fashioned books and paging through them trying to find salvageable stuff and so on, trying to work out what changed, what didn't, wishing I had 5E stats for stuff, taking photographs of maps and all that.