The 3-4 takes time
Patience is a virtue, unless you're an NFL fan. The Dolphins hire a new offensive coordinator, trade for a new quarterback, throw together a new offensive line, and fans are astounded, no check that, outraged, that the offense is not performing at a playoff level a couple weeks into the season!
We hire Dom Capers, a guru of the 3-4, to run a 3-4, a defense that has its own complications and intricacies, which require time to learn and adapt to, and we expect the Blitzburgh Steelers from the get-go. The problem isn't the 3-4, or Mularkey's playcalling on offense, or Culpepper, it's the expectations, which are not realistic. The problem is the total absense of patience, or foresight on the part of petulant fans.
Teams that win in the NFL have continuity, they have had time to play together, and apply their systems to their personnel. The 3-4 is complicated, and it requires time to install and run properly, there will be a learning curve, would you rather begin that learning curve now, or postpone it another year? We will obviously have some turnover on the defensive side of the ball next year, and in the future, but several guys on the defense have a long future ahead of them, so the sooner they begin play in the system the better they will be in the future. It's a lot easier to get two, or three new additions on defense to adapt to the system then it would be to get the whole defensive unit to adapt to it at some future time when we supposedly have the correct personnel to run it. How are we supposed to know who is and isn't capable of playing within a 3-4 if we play a 4-3?
The Steelers have been running their particular version of the 3-4 for 15-years. The Patriots have been running their version for 7-years. When Belicheck got to New England, McGiniest was used much as Jason Taylor had been used his entire career in the 4-3. Tedy Bruschi was no more the prototype 3-4 linebacker then Zach Thomas is. The Chargers have been running their 3-4 for 5-years now. The Cowboys installed their version last year, and had a lot of problems with it, now they're playing it at a high level. The benefits of the 3-4 long term far outweigh any short-term pains, which in any case are highly overstated because this defense, even while supposedly struggling has been very good.