The thing about Magic is that they do innovate new game pieces that add more toa. Game: FunkoPops are cute, but they only take up space as decoration. Also, Magic mostly makes their moneybfrom casual players, primarily buying at big box or drug stores...
And this is actually still pretty usable, since it covers a lot of specific areas that are not addressed by the new more big picture book, other than Icewind Dale, Baldur's Gate and the Moonshae Isles specifically.
I will believe the decline started earlier, but I learned to read in the 90s so mostly I'm just talking about changes I've seen since ebooks took off when I was in College. Heck, there has been a precipitous decline in Mass Market books juat in the past half decade it seems like.
And as you say, Amazon pricing was probably juat as much as detriment: I wouldn't be surprised if a lot of the folks who transitioned to buying direct had been prior Amazon customers for D&D products.
I think Dragon Delves is of pretty good quality, as an anthology all the modules are unburdened by the need of trying to serve a larger plot which tends to be the Achilles heel of a lot of WotC Adventure books. There is one Dungeon-focused module per major Dragon type, each by a different...
I think a more apt comparison is between recorded music and live music. Has recorded music changed the way people interact with live music? Yes. Will it ever fully replace it? No.
Oh, yeah, it isn't "real information" in that sense.
Nevertheless, books released after Behond starts direct physical sales are selling less, which would match with statements about the success of direct sales for WotC.