GTA V is one of the best selling home console games of all time because of its single player. Its online was broken for so long and it was still selling ridiculous numbers in spite of that.
Then there's something like The Last of Us which was a new IP that shipped over 7 million units on PS3 alone because of its single player experience. The multiplayer, from what I have heard, was great but no one was looking to buy the game for the multiplayer.
Most recently The Witcher 3 sold over 6 million units in 6 weeks and that has no multiplayer/co-op.
All developers and publishers need to do is make great games filled with adequate content at the very least and market them effectively and they will continue to sell games.