Firing Ken Whisenhunt after six seasons wouldn't make sense in the absence of a strong alternative. Reid would be a strong alternative. He would bring credibility to any organization. His time was up in Philadelphia, but he remains relatively young at age 54. This hiring would make sense. The team would not be taking a step down from Whisenhunt even though Whisenhunt's Cardinals were riding a three-game winning streak against Reid's Eagles, counting playoffs.
I think Arizona could instantly become a playoff contender by hiring Reid, re-signing Kevin Kolb at a reduced rate, drafting a quarterback and signing Michael Vick as a short-term starter. Reid, Vick and Larry Fitzgerald would combine to give the Cardinals more star power than they have had since Kurt Warner was behind center.
Reid's background is on offense. Hiring him as head coach would allow the team to keep Ray Horton as defensive coordinator provided Horton doesn't get a head coaching job elsewhere. The Cardinals could do much, much worse than having a Reid-Horton coaching combination.
Reid had control over personnel in Philadelphia. I'm assuming he would not take a job with the Cardinals without having significant personnel control. The Eagles usually had a good feel for when to part with veteran players. Reid presumably informed the team's judgment in that area. If Reid takes the Arizona job, former Eagles and Cleveland Browns executive Tom Heckert would be an obvious choice as general manager. Under that setup, team president/owner Michael Bidwill would keep a low profile.
The Eagles were a model franchise during Reid's tenure. Turning the Cardinals into Philly West sounds like an upgrade.
The Cardinals have never had a television blackout for a game at University of Phoenix Stadium. That streak would have been in jeopardy, in my view, had the team stayed the course after finishing 5-11 this season. Reid would get fans excited. Adding Vick would generate national buzz in the short term. For all his struggles recently, Vick was exponentially better than the quarterbacks playing for Arizona most of this season.
There would be other options at quarterback. The fact that Reid was with Vick and Kolb in Philadelphia doesn't necessarily mean he would want to proceed with either one in Arizona. He unloaded Kolb to the Cardinals, after all. Vick and Kolb would know the offense, at least. They would represent a significant upgrade from Ryan Lindley, John Skelton and Brian Hoyer.
An NFC West with Reid, Jim Harbaugh, Pete Carroll and Jeff Fisher sounds good to me. Call it a Mt. Rushmore of coaches still seeking Super Bowl rings.