jmartkdr2
Hero
I think a big factor in these sales is player options. SCAG had exclusive cantrips, Eberron had the Artificer, and those kept those books in the conversation and made them useful even if you don't care about the setting. The MtG books offer all new races, at least, and they also bring in new audiences. A pure classic setting/lore book is the weakest option, IMO.I don't doubt it would sell (all D&D 5E books seem to sell very well) but I'm not confident that a 5E FRCS actually would sell better than Planescape, Dark Sun or Dragonlance. Looking at the Amazon sales right now, the SCAG is roughly neck-and-neck with both Theros and Ravnica (#21, 22, 23). But Eberron is running way ahead (#14). Makes me think that people are more interested in D&D legacy settings that explore their own niches (Ravenloft is of course releasing next week, but it is sitting at #1 right now).
I've heard folks say that if the SCAG was more like FRCS it would have sold better, but I'm not so sure about that. I don't know which FR fans would buy a 5E FRCS but not the SCAG.
This is why I think a book with Planescape is likely, but a pure Planescape book is unlikely: they could include Spelljammer, and magical airship rules will sell books to a lot more people than Bariaurs.