What does Carson gain by taking a stand and promoting change within the Bengals organization at this stage in his career? Not a thing. Because by the time it materializes, he'll be at the very tail end of his career and won't be able to take advantage.
As Hoosier said, I think it's a simple matter of him looking at his options and prioritizing them based what he found most desirable: first is to be traded, second is to retire, third is to play for the Bengals. Since SoP will never trade Carson, option two it is then. I don't think Carson being heroic nor is he being a coward by retiring. He is putting himself first, though, and I can't blame him for it. In in his situation, I'd make the same choice.
SoP never puts his players above making a buck, so why should the players put SoP and the organization above them?