Co-op.... the fact that most sequels sell half of what the original game sold unless it's offer radical changes. PoE2 offered ship combat but it was fairly minor and cosmetic. The rest was 'more of the same', which after PoE + 2 expansions was a very hard sell.
How do you explain D:OS2 success? Wasn't it also "more of the same"?
https://gamefaqs.gamespot.com/boards/687020-pillars-of-eternity/72017511
Q: was wondering if i could play [Pillars of Eternity] with a friend?
A: Divinity Original Sin is the game you want.
D:OS 1 also had co-op, so your argument is that the existence of a co-op makes a sequel sell more, although it's something present in both? If not, I didn't understand what you mean.
If "sequels always sell less" because they are "more of the same", and D:OS2 was literally more of the same (and it was, co-op included), either the premise is flawed or the nature of the co-op has something that modifies the effect of the premise.