Your overlooking the change in coaching in Indy and the fact that when you lose arguably the best QB to ever play after over a decade of stability @ that position, obviously your're going to see a regression in performance.
Another example, look at San Fran. One year with Singletary, they are putrid. The very next year, after adding Harbaugh, they come within a game of the Superbowl.
Having a Luck, Brady, Manning, or even guys like Matt Ryan and Big Ben give you a better chance to win obviously, but coaching is key.