The culture is a bit different these days. People have no problem buying a coop game and quitting before the first act is over, moving on to a different game with their friends next week. No, it's definitely correct to say that, in the case of Divinity OS2, coop was a big selling point. Many streaming groups animated many, many people to try the game with their friends. That it is too long to realistically finish is obviously true; this target audience, however, doesn't mind. Just having been able to play together for 4, 10 hours is enough for them to have justified the purchase.
You always look at things with your sensible eyes, failing to realize that basically everyone under 30 these days has serious ADHD and can't focus on one thing for too long, so they are fine with playing a game for an hour and then dropping it.