Yep, I agree.
Without any meaningful PvP, decent modding support or an engaging gameplay loop in the campaign I do fear for the games longevity.
I mean looking at the campaign maps in PVP, some of the setup locations for specific Infected to engage were amazing, walking around corners being paranoid about whether you were going to get charged off a cliff, or a smoker pull off a ledge is half the reason its so good. Nailing that dude that though it was clever to try and rush the map just felt so satisfying.
Also look at the sheer number of utterly amazing custom maps available in L4D2, all of which can be played online with a simple click of a button in game. Maps like Suicide Blitz 2, Warcelona and Daybreak only add to the experience overall and serve to show why its still played daily by 10k+ players.