Wanny did 4 things to improve the roster he was given by Jimmy Johnson. He drafted Chris Chambers, Randy McMichael, signed Ogun and traded for Ricky Williams. Considering Madison, Taylor, Thomas and a vast majority of the Defensive foundation was drafted by Jimmy (and Timbo by Shula), Wanny did LITTLE to improve this team.
In order to improve a team after 5 years of coaching, your draft picks ultimiately must pan out. As you can see, he only actually added 2 starters via his drafts. That's a very poor track record.
Wanny has never been able to accumulate talent based on his own judgement. He was merely someone to hold the fort down after Jimmy left. Nearly any legitimate coaching prospect in the NFL could have managed the same results given the stellar defense Wanny was giftwrapped.
Jimmy gave us a 5 year window to be competitive for a Super Bowl. Wanny's inability to improve on his mentor's foundation is a shining example of giving the incompetent every opportunity to succeed only to listen to excuses.
The only thing Wanny was right about in his entire tenure was quitting. It WAS the right thing to do.