Something else to keep in mind is that "player movement" is exactly what has caused the quality of play in the NFL to decline so much over the years -- there's simply no chemistry on any team these days.
My guess is that the NFLPA would love to consider changes in the terms of the salary cap -- because it turns out that it wasn't all that it was supposed to be. A team that signs a couple of high-priced free agents is often forced to scale back on their offers to other players, and the end result is that salaries for key players have gone up while salaries for most of the NFLPA members are actually quite low -- especially when you consider that they don't have guaranteed contracts. The NFL is the only major professional league that allows teams to "re-negotiate" the terms of a player's contract without any input from the player himself. That's pretty ridiculous, I'd say.