It's still not that much coop.
If you're really skilled or use bugs/glitches on map or whatever you can make it
If you get on a glitch spot on a map a specimen can still get there. KF-Departed is a example of this.
Yes, you can be really skilled. Although when your a Support Specialist, and you've got a Husk out there along with 4 Scrakes coming your way, your gonna' need a Sharpshooter against that Husk so he doesn't cap yo ass nigguh.
Now, in L4D, when you get pounced on, or dragged away, or whatever, that's generally the only thing that requires teamwork in the game. Everything else is possible by yourself. All you really need to clear out a horde on expert is a Auto shotgun. Tanks are easy to avoid (Especially in L4D2. In L4D2, Tanks are wussies.) and Witches are even easier to kill. The only SI that's a pain to kill by itself is a Jockey.