What can we learn from Davis' experience? Mostly, the same thing we can take from most coaches' records:
Players matter more than schemes. Period.
The 49ers defense improved markedly after Davis left. Is that possibly because the team added Patrick Willis, then Justin Smith, then Dashon Goldson, then NaVorro Bowman - the nucleus of the defense that just played in the Super Bowl - over the next few years?
The Cardinals defense was solid in 2009, Davis' first season as coordinator. He lost linebackers Karlos Dansby and Chike Okeafor, and the defense regressed. But the biggest reason for Arizona's decline from 10-6 in 2008 to 5-11 in 2009 was the departure of quarterback Kurt Warner. Head coach Ken Whisenhunt fired Davis, but it's not as if the Cardinals ever returned to their 2008 Super Bowl form.
It is all about players, which puts the real focus not on Davis and the expected shift to a 3-4 scheme but on general manager Howie Roseman. There are some pieces in place for Davis - Fletcher Cox, Mychal Kendricks, Brandon Graham - but Roseman is going to have to find at least five new starters through the draft and free agency.
For the first time in a generation, the Eagles are going to put a premium on play-making linebackers. That's something to look forward to.
Does any of that mean Davis will be a good defensive coordinator? It does not. It is, in fact, impossible to believe that he was the best candidate available for the job. That makes his hiring consistent with a franchise policy that one hoped Andy Reid took with him to Kansas City.
Despite having no NFL background, Kelly has managed to tap directly into the league's old boy network for both coordinator jobs. Davis' father, Bill, was a longtime NFL coach and executive, including time with the Eagles. Shurmur's uncle, Fritz, was a legendary defensive coach. You might have expected Kelly to bring in some new blood.
A little diversity wouldn't have hurt, either. The league's scarcity of strong minority head coaching candidates stems in part from the lack of minority coordinators. And that scarcity stems largely from a pattern of hiring white-guy retreads from the network, which is precisely what the Eagles have done here.
The delay in hiring Davis, who was available all along, suggests Kelly had at least one eye on someone else - perhaps an assistant from the Ravens or 49ers, or maybe a college coordinator such as Georgia's Todd Grantham, who is scheduled to interview with New Orleans.
But there was nothing about Sunday's Super Bowl that compelled you to rush out and sign one of the defensive assistants. If you want busted coverages and clueless cornerbacks, just cue up the Eagles' 2012 highlight reel.
As for Grantham, his only experience as a defensive coordinator in the NFL - with Cleveland, of course - ended after the 2007 season, when his team was ranked 25th against the pass, 27th against the run, and 30th in total yards.
You could make the argument that defense is so degraded in the modern NFL, it doesn't matter who the coordinator is. The counterargument, of course, would be the mess the Eagles made last season. The rules may favor offense, but defense still matters.
Davis may be a less-than-glamorous choice. He may not have been Kelly's first choice. None of that will matter once the games begin. Then it will be up to Davis.
The bar is set very, very low.
Contact Phil Sheridan at firstname.lastname@example.org. Follow on Twitter @Sheridanscribe.