I'm all about the single player experience, and I'd say there's definitely been a shift towards online multiplayer. There's no doubt that publishers see the success of COD, which is pretty much entirely based on its MP mode, and they'll want a piece of the pie, so you get endless clones, but the publishers aren't the only ones to blame.
You look around major gaming websites and you see opinion pieces from editors demanding online multiplayer or co-op shoe-horned into games that were a success because of their excellent SP modes, and I can't help but think, why would anyone want to see an excellent game have its sequel tarnished by the developers being forced to spend time and money on something that doesn't fit in the game (so the difference between Bioshock and Bioshock 2)? It makes no sense...