They aren't going to release two adventure compilation books in quick succession. Nor are they going to release a compilation book set in Eberron. It's not portable. They will always go for a GenericFantasyland setting for adventures, so that they can be ported to any setting. It's just a matter of understanding the market and accepting that WotC's job is maximising profits.
Well, perusing Amazon, Candlekeep Mysteries are about 400 in all books, so, I'd say, so far anyway, it's selling pretty darn well. Any RPG book that breaks the top 1000 on Amazon is moving a decent amount. Of course, it's still early days. Ask again in three months and that's probably a better sign of how well it's doing.
And sometimes we take turns! To be fair, the sussing is wrong morenoften than right...but it's a fun rush when we piece something together correctly.
I'd actually say everything that's been tested so far this year isn't so far out: Races usually get public testing pretty far down the publiahing pipeline (or even after getting published, as with Eberron). Whatever the heck is up with the Fey stuff I think we'll see by end of year: either as part of a Fey Adventure, mixed with the Dragon stuff, or the mysterious fifth book.
As we know from the schedule, it will be within 12 months of the UA, which was March. So it will most likely be this year. However, we know 21/3 is an adventure, so is unlikely to include four new lineages; 21/4 must be the dragon book, because the first UA related to that was October last year, there is plenty of dragon related material without having to lump fey stuff in with it; ergo the fey stuff pretty much has to be 21/5 (I.e. the Christmas release). Assuming it doesn't go in the can.
Well, they put player material in Princes of the Apocalypse: anything is possible. I rate that as low likelihood, but it's possible.
A known internal playtester commented on the thread for the blog post and said that since the infamous Elemental Evil cancellation that no book has come through Alpha testing and not been published in some form. Their track record in the past few years for UA suggests that if a UA gets put out, the book has made it past the development hump, though individual options, such as mass combat systems tested for Xanathar's Guide or the Vishiano Race for Ravnica, can get the axe.
That never made it into a UA. There's also goliaths in RIME OF THE FROST MAIDEN. But also untested
