I used to think that a single player campaign would be nice, but after playing the multiplayer campaign, I don't think a singleplayer campaign would have been very good.
Within even the very limited boundaries of what they could do in a multiplayer campaign, it turned out to be very clunky and forgettable. They had very little room to work in plot elements, so they should have cut things down to the bare minimum instead of involving so many characters, and kept the plot simple. Instead they put in too many characters leaving little/no time to get to know them, and spent nearly all of the voice time on explaining why we're going from one map to the other for "reasons".
When you've got so little room to work with, a "less is more" approach needs to be used, like in L4D1 and L4D2. Plant the seeds of an interesting story and let the player fill in the gaps with their imagination.