Jimmy Johnson won two Super Bowls in those unique circumstances. The process that has resulted mostly in mediocrity is all Garrett's. He's wasted talent for years by trying to slow-build a winner, which has either left us without a defense when the offense was good enough or resulted in talent we were relying on leaving because of injury or age before we could complete the process.
For Garrett's plan to work, one side of the ball has to be great to overcome the need to build the other side. The offense has been close at times, but was held back by the scheme/play-calling needed to put it over the top. The defense might be close this year and get held back by offensive shortcomings.
Winning a championship is a combination of having the right players, having the right coaching and luck. Having the right coach goes a long way toward getting those first two things.