The problem is very simple here:
When Whiz took over this team nobody knew that Kurt still had this type of game left. With that in mind, Whiz began to build Pittsburgh West. In doing so he wanted to use a 3-4 defense. He also wanted a run heavy O that pounded defenses into submission and beat them through the air when they weren't expecting it. Essentially, he wanted to build a team whose strengths would be running the ball and stopping the run.
All of that is great, buuuuuut since we found what Kurt was able play at this level we have been throwing the vast majority of the time. When you throw with success it forces your opponent to throw. The 3-4 scheme's weakness is playing a good passing team. Look at what Warner did to Baltimore in that one half of play a few years ago. Look at what Pittsburgh did to GB a few weeks ago, what we did to them a few weeks later and what we did to Pittsburgh in the SB. Finally look at what good passing teams (and some who weren't) did to us through the air. The problem is that we have a D built to stop the run, but our O forces teams to pass on us.
Now Billy Davis does suck. He has shown that throughout his career. If you were expecting anything decent out of his D than you must not have watched any of the SF defenses he coached. I think his hiring was no more than Whiz trying to see what some consistency and stability would do for the D after Butler rejected him. Now that it has shown to be be irrelevant I expect to see him fired shortly. However, as long as Kurt is QBing this team I don't expect any 3-4 out of us to be all that great. Passing teams' best shot is a 4-3 predominate Cover 2 IMO, similar to what the Colts, Vikes, N.O., and Tampa employ. It gives you smaller faster players who can excel in coverage as well as pass rushing for their positions. Obviously that D's weakness is that they can be bullied off the ball.
With all of this said, I think Leinart will be the man next year and we will begin to run the ball more. That in itself will improve our D. I like the 3-4.