The money would be better used elsewhere.
The loss of Ryan Shazier leaves a huge gap in the middle of the Steelers defense. Although none are of the same caliber as Shazier there are several players who could fill this gap. Giving Bell the contract he wants would make it virtually impossible for the Steelers to make a play in free agency.
Young run stuffers like Todd Davis, or Avery Williamson could really improve a run defense that was very suspect after Shazier went down. A large contract for Bell could also cause cap casualties elsewhere.
Mike Mitchell, and Vance McDonald are both in danger of being let go. While, Mitchell is getting older, and McDonald is injury prone both have proven their value. McDonald showed in the playoff game how impactful he can be when healthy. I would much rather solidify the rest of the roster than pay Bell millions more than other running backs.