Again, there's an elephant in the room regarding Deadfire's dip in sales that no perception of quality can account for, I feel. The dip the franchise saw in performance wasn't a 10-20% or a 50% even, or a lack of continued interest or whatnot. From what I understand, the first game in the series has already sold about 2 million copies whilst the new one has struggled to make it to 400k. That's a 70-80% drop right there, and one for a game that, despite assumptions of the contrary, is generally rated quite positively across all user aggregate websites out there, including a 7.8 metascore, an 84% Steam user approval and a 4.4/5 GOG rating.
Moreover, let's consider some notorious sequels to popular franchises:
Dragon Age 2: roughly 2 million copies sold by August 2011 (down 50% from its predecessor)
Mass Effect 3: roughly 1.5 million copies in its first month (100% up from its predecessor), roughly 6 million total (down from Mass Effect 2's 7 million)
Mass Effect: Andromeda: an estimated 2.5 million copes sold across its lifetime (down 60% approx from Mass Effect 3)
BioShock 2: 3 million copies (down 25% from the 4 million sold by its predecessor)
Fallout 4: roughly 14.9 million units sold (up 50% over the next highest-grossing Fallout game, Fallout 3)
Batman: Arkham Knight: 5 units sold in the first four months (down 17% from Arkham City by the same point)
And more. It's arguable that all of these examples were met with much more vitriol from the gamer community and proved to be way more divisive if not outright trashed by the fanbase than Deadfire, yet the latter's drop far exceeds even the worst ones here. What's more, some games that were deemed disappointing relative to their predecessors even went *up* in sales relative to their previous entries as well, which goes to show how much the insular jabber of the diehard community often matters in these things. To be fair, Deadfire is a smaller game and thus likely more exposed to such talk - but again, when the talk wasn't even that negative to begin with, it's hard to point out to it as a major factor in the drop in sales.