My take as an Iggles' fan: I loved, LOVED, the hiring of Kelly. It was about going from fairly consistently good to great. Andy Reid was like Bud Grant, there was just a level beyond which he couldn't reach. Lurie let him stay on one year too long as even the players had started tuning him out. Happens with everybody.
The gigantic mistake was giving Kelly the keys to the personnel car. Someone on this thread talked about giving him time to fix the o-line. But Kelly chose to not even draft an OL despite the lack of depth, and Kelly chose not to even offer Evan Mathis an extension. The starters don't suck, but their complete lack of depth was exposed. Kelly in essence created the OL problems, so how much confidence could we have that he'd correct them?
The other major problem was that Kelly turned out to be a total d-ck to the players. There's no need to coddle players, but they ARE grown men and many of them are prima donnas. Kelly chose to get rid of pretty much everybody who didn't kiss his ring. Lurie realized he'd made a huge mistake, and he decided to cut his losses. Could Kelly go to Tennessee, reunite with Mariota, and have success? Possibly. He succeeded with Reid's players for awhile.
But he'll need to change some things. His uptempo offense left his own defense with their tongues hanging out. The Iggles' defense was good the first half of the season, but they were totally gassed in the second half. And, most importantly, he'll need to really learn the lesson that system does not trump having talent. Talent is what's important unless perhaps you're Belichick, and I think that cheating is his ace in the hole. Plus, he needs to strongly improve his interpersonal skills. What he does works in college, but so far it hasn't worked in the pros. He completely lost the team. And, just like that, he was gone.