While I do agree that some forms of multiplayer would be great with KCD (like coop battles), I think you are disregarding the game too fast for the lack of it and generalizing that people will not find this game interesting.
I love the single player of games, especially if the story/setting is good like in KCD. Building multiplayer will take time and push the release date further. Also, without a story I do not think I will enjoy the multiplayer at all. For instance, I’ve finished GTA V without playing other games but did not spend more than an hour in multiplayer mode.
I am not saying everyone should like the single player. But for me this game is all about the story telling which is much better when done in single player. The inclusion of a multiplayer portion of the game, even if the developers want it, would not be feasable to build for act I.