Single player games aren't dying. Some do get tainted by multiplayer, or get lazy with the campaign since co-op should fill in the fun.
For example Gears of War, the parts where there are 2 obvious paths next to each other completely broke the immersion and turned it into an amusement park ride. Or a game like Left4Dead or Dead Island which is not suited very well for single player.
It grates the other way around as well, shoehorning co-op into a single player campaign. For example DKCR did not work well in co-op.
Either make 2 separate tailor made campaigns or be clear about it. 1 player, 2 player or 2-4 player. Not 1-4 player with half-baked single player experience or awkward multiplayer.







