There's no way that trade would go. In his first four seasons (age 21-24 seasons), he picked up nearly 6,000 yards; in his last four seasons (age 25-28 seasons), he has gained less than 4,000 yards. Two of the last four years ('06 and '09) he was limited to around 500 yards due to injury; such is the nature at times of playing in the NFC East.
One could make the argument that trading him to a weak division could provide a possible resurgence for him, but he's not likely to be a Jim Brown (over 1,500 yards in 14 games in age 29 season) or Barry Sanders (over 2,000 yards in age 29 season) when it comes to producing at his age, especially with the knocks he's taken lately. That would be too far a reach for most teams (we're not dealing with the Raiders, after all). As was noted before, Cromartie is in a much better position to have a renaissance, being younger and at a position that requires great speed, but not the punishment of running back. Why give up a potential decade of very good production, for a couple of years of good when healthy, with said health in recent short supply?
The Bronco organization knew they would get the better of the trade when they made it. They could always throw another running back into the mix to get those yards, while upgrading from LENNY WALLS to CHAMP BAILEY! I didn't like the trade when it was made and still don't, even though Portis has had some productive seasons. Champ was too good to let go like that, and Portis is a drama magnet.