It's another to say he's less of a fit than Darren Sproles. The only issue that has ever kept the unique small back from being more than a special teams and/or pass-catching Pro Bowler is that you don't want to give a guy his size too many touches, for fear that you'll burn him out. He's also got some mild issues in blitz pick up, and probably isn't your best choice in short yardage, but on a per touch basis, there haven't been many more effective players on the planet than Sproles. (Oh, and he's also pretty old for an NFL RB, but what the hey, Sproles is already on the outlier range for everything else in his career.)
The biggest issue, to my eyes, is Kenjon Barner.
To call Barner street meat is, well, simply a moment of accuracy. He was the 182nd pick of the draft in 2013, a sixth rounder for Carolina. At 5'-9" and 185, he doesn't have true NFL size. He's also not devastatingly fast, honestly, or even all that shifty. He's been in the league for three years, but didn't get off the Eagles practice squad in 2014. If it weren't for a couple of special teams touchdowns in the preseason, he might have lost his spot to Kevin Monangai, a free agent from Villanova who had better measurables, and who hasn't resurfaced in the league after getting cut. He might also just have his job in the NFL due to Kellly's nepotism for Oregon guys.
And maybe I'm making too much of a small sample size -- Barner has 23 carries for 106 yards this year, a 4.6 average, with no touchdowns and one pretty terrible fumble -- but if a guy like that can be more productive than a high-priced free agent, and a guy who had the best numbers among any RB in the league last year?
Well, that's not the system. It's the player.
If the Eagles release Murray -- an idea that seems to be gaining in momentum for reasons that seem to have more to do with preserving his fantasy league value than any real-world utility -- I really don't think he just signs to some other team and is all good again. The Dallas offensive line in 2014 made Joseph Randle look good, too. Randle didn't go much with his touches this year, got released, and hasn't been seen again. Maybe that's just because he's Froot Loops, but still, the NFL tends to put up with that sort of thing if you can help a team win. Finding that kind of historically good situation to be a RB again is really not a given.
More to the point... what team has a demonstrably worse RB situation, that Murray actually improves, right now? What team would prefer to address that issue in the off-season with an aging RB with a high price tag, going on his third team, rather than a fresh college kid with much greater opportunity for explosive gains?
Remember, Murray's longest run from scrimmage isn't past 40 yards in his whole career. That doesn't speak to a true premium talent, even during the glory years, prior to Dallas giving him 450 touches in 2014 because they knew they weren't bringing him back. People want to talk about Murray's one-cut ability, his power in the hole, his ability to fall forward, his ability to get better with more carries... and, well, none of those talents are particularly rare or important. A back with east-west speed can do one-cut decisiveness. Falling forward is a function of offensive line play. Getting better with more carries is of small importance, especially if it means that you toss away the ability to develop depth in the event of injury.
There's a reason why smart teams don't pay top dollar for RBs, or sign big names. That reason is because it's a terrible investment. (For sake of argument, we won't even get into the sideshows about how Murray's been shying away from contact, wearing ski caps on the sidelines, and going to the owner to ask for more touches. Even if it's true, it doesn't really matter for the point of staying focused on actual game.)
The first rule of terrible investments is, of course, do not make them.
The second rule is if you do make one, cut your losses as soon as humanly possible.
And to anyone who thinks that Murray will just go back to Dallas and return to wreak vengeance upon the Eagles?
Well, what part of the 2015 player don't you want on the Cowboys in 2016?