The only way one of the top 4 running backs fail is because of injury, or Bell could sit out some time. I wouldn't put it past him with the Steeler's kind of screwing him around with this franchise tag thing. I get it from a business perspective, but man the franchise tag system really screws over positions that don't carry a large salary at the top.
Personally, Fournette and Mcaffery are right there in the top 5. I would bounce Bell because of the uncertainty. Someone out of the top 4 has to end the season as a disappointment. Any reason other than injury, Bell is the only one I can place logic to. I am nervous about Kamara, Cook, and Hunt. I think they all could be in for some regression from an efficiency standpoint, while I think Fournette will improve on his efficiency.
I am more of a David Johnson 1 overall person myself, but the fact remains, his offense is just not near what the Rams offense is. Take Gurley. He is durable. His team is awesome.