Gamers are always so demanding...no matter how good and enjoyable a dev made a game there will always be haters ; specially if the dev is obsidian , I am very sure that if Bioware were the dev that changed the things Obsidian did with the franchise they were praising them.
To the OP why you bough the game after play the demo or read the lot of info about the co-op online the devs confirmed in old interview/recent interviews.
The main focus on DS3 is the story not play with nannies to watch my back online.
the co-op is a bonus..at least this RPG support it , not like DAO , DA 2, ME 1 and ME 2.