Does every linebacker in the league need one of these guys?
Yes, pretty much. Very few MLBs have had success if the guys in front of them aren't doing their job. It's not to mean a MLB is solely dependent on the the D-line, but it's like the relationship between a safety and corner. A good safety with a pair of average corners is not going to be as good as he is. With good corners, a good safety is going to be significantly better because it will allow him to do what he does best (stuff the run, play the pass, etc.). Ray Lewis would not be Ray Lewis if he played for the Saints. He'd still be good, but wouldn't be close to how good he is now.
The way you guys are talking it isn't Hartwell's fault he sucked, it is the defense, the personel, the play calling, the coaching staff, etc.
I don't think Hartwell sucked at all. His problem was that he didn't make enough plays against the run. Truthfully, if you think Hartwell sucked, hten Brooking and Demorrio should be right up there with him. Both have played better than Hartwell, but both haven't been having a great impact vs. the run either. I'm not going to say Hartwell doesn't deserve a share of the blame, but heaping all our run defense woes on him is unfair. Basically, nobody has been doing there job. Sporadically some of our guys will make a good stop, but it hasn't been consistent. Hartwell just comes under the crosshairs because he's the guy in the middle. It is all those things you mentioned (personnel, play calling, coaching staff, etc.) that is contributing to the problem. One of the few positives that came from Wade Phillips in 2003 was that he somehow managed a way to stop the bleeding on a terrible defense. They didn't become good at any point, but they ceased being terrible.
Also how much of this is the fact that in his first year, Mora and the defense exceeded everyone's expectations. A general rule of thumb in the NFL is that if a unit, team, or player overachieves (as ours did last year), they will fall back to Earth the following year.
If McKay was right (and I am not saying he is) then Hartwell didn't live up to his contract and has so far been a bust.
As Angry said, its too early to judge. If the run defense continues to be a problem throughout the rest of the year, it will prove that Hartwell is not a bust and the Falcons as a team are struggling. If it suddenly improves, then it will mean Hartwell shares a heap of the blame. BUt then again, you have to factor in how much of that change is due to the coaching staff switching things up? With Hartwell gone, the coaching staff HAS TO address the run defense now, if they were ignoring it up to this point.