I wouldn't bundle all these titles together. An argument might be made that most DO1 players didn't finish the game because the second half was objectively much worse. Or that, as I said above, PoE1 and XCOM rode hard the credit deriving from the old classics they claimed to be inspired to, and that works only if the game you release keeps its promises. Or that Banner Saga was overhyped as hell (mostly due to the gorgeous graphics and atmosphere). Or that Grimrock 1 projected the aura of a worthy heir to classic blobbers, and it came short, breaking the illusion in similar fashion to PoE1. Etc. etc.
It's much worse than this. Nearly 37% of all registered Steam games (that’s 781 million titles) have never been played. Read
this and
this. How can we take these numbers at face value if nearly half of the games are never played? Steam sales and humble bundles are part of the reason, but I have the suspicion that players buy expensive games on release that they never intend to play just because they feel the need to be on the loop of the latest hot shit, because they are gamers and that's what expected from that, etc. It's easier to buy games and feel good about yourself even though deep down you never had the intention to play most of them. Typical consumerist behaviour.
Only 5%(!) of players finished DO:S, yet the game and the sequel was a success. This is an obvious indicative that these numbers don't reflect how many actually enjoy these games. You talked about the second part being a chore. This didn't affect the reception of the second game. How come? I also bet that very few players played the game after the first two hours because players are being induced to buy and say positive things about certain titles they don't like and don't want to play. I remember someone defending DO:S2 here on the Codex, but the dude didn't even played the damn game. “I have the game on my library and I didn't play
because I don’t have the time, but it’s a good game!”. Pure rationalization and herd mentality.