Well, I think the Panthers organization figures that those would have been Steve Smith's numbers had he not been hurt. They must have also seen something in Colbert. Because of that, they did not want to pay Mushin $10 million in bonus money and give him an insane contract hike when they probably wanted to phase him out of the offense anyway and go with the younger guys. The running backs should also be back at full strength next year which will make that team a much more balanced offense. The only concern I have is that both Smith and Colbert are smaller guys, and most teams have at least one strong and tall WR that can make those "jump" catches. They will miss that next year, for sure.
Muhammad has been a good receiver for dang near his entire career. He was forced to do more in the way of pass catching last year than he ever has before, and he stepped it up. You're right that he did do a lot of good blocking for the team as well.
Whoever gets Muhammad will have a very solid NFL player. The Eagles and Vikings will be looking to add this guy for sure.
Somehow I don't see the Steelers trying for him...they aren't big into paying out huge dollars for free agents (but I could be wrong on that