Fitz is a Game Changer.
I can see your point with Fitz. You could get alot of pieces for him. It's a borderline argument. But I side with keeping him and here's why. Fitz is a true game changer. One of the handful of guys in the NFL that you must gameplan for. The type of guy that keeps coordinators up at night, demanding double and triple teams on every play. These guys win playoff games for you if used properly. Look at the playoff game against Atlanta. It was a joke. Warner was throwing jump balls and Fitz came down with every one. It wasn't even fair.
How many true game changers are there in the NFL? What 10? Maybe 12? Most teams don't even have one. If you do have one of these 10 or 12 guys, you break the bank to keep him on your team. Thankfully, the FO was smart enough to realize this. The NBA is the same way. Dwight Howard's worth once came into question. The correct answer was 'the whole team'. You would trade the entire Orlando Magic roster in order to keep Howard. And the same goes for the other 10 to 12 NBA Game Changers, with some exceptions. And the same goes for Fitz. Except not the entire 53 man roster of course. But surely an NBA sized roster of 12.
Edit:
I was just thinking, how many true game changers have the Cardinals had in their history? I can think of 2, maybe 3. Fitz, Warner, and maybe Otis Anderson. (Not sure about Otis. That was a long time ago, so I was too young to fully appreciate Otis.)