I haven't touched L4D since before L4D2 came out, and I hardly played L4D2, but what I found was that the further we got from the original launch, the less people there were on the servers, and the more dedicated those remaining were. It felt like you had to stay very active or get left behind, such was the tactical/practice element.

So while I liked the team-based nature, I found it actually didn't help longevity - for me, at least. Well, more relevantly, this is also the case for my friends - individually I'm a poor data point as L4D was the last game I took any real interest in at all.