It was not a great signing because 1) you don't pay running backs and 2) you don't pay running backs.
This.
Plus, are we sure Bell is still good?
Last year, Powell was more effective than Bell. This year, Gore was more effective than Bell today. The Steelers replaced Bell with James Conner, who was more efficient than Bell when he was healthy.
Are we sure Bell is still this elite running back? I'm not saying he is worse than those guys, but neither the Jets nor the Steelers saw any drop in production when he was gone.