Parmandur
Book-Friend, he/him
2,500 out of all books is steady regular sales. And we know from the recent Bookscan leaks that those three books are still regularly selling week to week. That and with how WotC looks to be referring to those books in the new Core rules (based on the text that we have seen), no particular reason to suspect they will be out of commission any time soon. SCAG has been in print and actively selling for over 8 years...meaning longer than the core books for any prior WotC Edition.If you look at the Amazon Sales numbers: they're not significant. If you talk to game store owners, you'll see that the sales for books like these drop off over time and become problematic. They are not selling enough to warrant shelf space, but you don't look like a professional store if you don't have the full 5E line available - and you take a hit if people find that you don't carry things that they expect you to carry, even if there is not enough turnover to warrant space on the shelf.
Sword Coast was once top 20 in the best seller list, but is over 2,500 down in the best sellers list - meaning it is in that wacky zone of trivial sales.
There are some great blogs from store owners, like the one from Black Diamond Games in Concord (CA), that discuss the challenges around Board Games, D&D, Pathfinder, Pokemon, Magic, Lorcana, etc... selling. It is interesting stuff, even in you do not ever plan to run a game store.