Quote from DefragI think calling the original 'barren' is way, way wide of the mark. Each campaign takes roughly an hour to play through; 4 campaigns = 4 hours of gameplay right there. Some single player games last 4 hours start to finish while lacking any kind of replayability.
so just because stealing is popular, it is automatically legalized? and even if that statement wasn't a logical fallacy, i still wouldn't expect this sort of dick move pulled from a company of valve's stature.
also the issues you mentioned are not reasons enough for a new game; they are reasons for an update to some of the systems used by l4d (you should know this as a programmer).
a clue towards the conception of l4d2 in the first place may be that the original l4d wasn't made by the core valve team, but by turtle rock studios (who valve decided to buy at some point for whatever ridiculous reason in their heads).