As you make so clear, the D was built when he got there, but he didn't have the resources needed to build a more complete offense. Despite that, he won a SuperBowl and made it to the playoffs two other times at Tampa.
It is also convenient that you completely ignore his successes at Oakland.
It's clear that one side of the ball, no matter how good, can't completely carry a team. The Vols this year have the second best offense (statistically) in the SEC, yet they are continually getting beat. The 2008 Volunteers were ranked #3 in the nation (tied with Bama actually) in total D, yet they still had a losing record. Gruden did less with more to get that offense efficient enough to win. Further, the Head Coach should get the credit and blame for their assistant coach hiring and rentention rates. Gruden benefitted from that D, because he kept Monte happy and on staff until the end of the 2008 season. Our current coach doesn't seem to have the ability to retain anyone but Cheney and Hinshaw.