Time will tell. I think a key coordinator can be all the difference. Look at Buddy Ryan in Chicago as an example - that team faded pretty fast after he left.
Super Bowl winners are teams that put it all together at the same time; ownership, coaching staff, players, and a little bit of luck. It is not at all suprising, once the team that managed to win a Super Bowl breaks up, that the resultant team doesn't do as well.
Also, after his Super Bowl with the Giants, Parcells took over three teams, which were 2-14, 1-15, and 5-11 the seasons before he arrived.