Its hard to ignore sales, looking at the best sellers every year, 15 of the top 20 sellers will have a multiplayer in their game, in theory multiplayer is a great idea that will make the game better and give it a ton of replayability.
However, thats only true if the multiplayer is fun, has enough people playing it, and had enough time and money put into it to make it worth while.
What we've been seeing lately is medicore single players mixed with mediocre multiplayer, creating a mediocre game, multiplayer when used correctly, is fine, but when its tacked on purely for the sake of sales, all its doing is keeping the single player from being as good as it should be.
But unfortunately, you see games like Vanquish, Mass Effect, and Infamous that are amazing, but dont get the sales they deserve because they dont have multiplayer. Its even gotten to the point that its a "Flaw" if the game doesnt have multiplayer, games had single player only for over 2 decades and they were doing just fine, why do we have to drastically change that?
Im all for innovation, i dont want to play the same game over and over, but if you are going to make a multiplayer, make it because you think you have a great idea for a fun multiplayer, then put the proper time and resources into that multiplayer to make it successful, afterall, if your multiplayer isnt any good, nobody will play it, and its a useless feature, unlike single player that you can pick up and play 20 years later.