I've been of the opinion for years now that Jackson got too big. When he was a young RB he had much of his mass in his legs and had much better burst and short area acceleration. He still can run when he gets a load of steam going, but he's not the same guy any more with all that topside muscle he's got going. The dude looks like a DE, he's absolutely enormous and that isn't helping him.
Anyway, I love the way Fish and Snead handled his departure, it was absolute class. They knew he wasn't the same guy and were smart enough to let him walk. The Falcons banked on him, which was a mistake.
Jax could still do the job with a dominant OL, but then again you could say that about most RBs at the end of their career. What he no longer does is make everyone around him better in that running game. It won't be much longer before the Rams are doing a sign and retire deal with him, IMO.