Well, technically, Pierce could opt-out, get a 6 year extension at 6 mill per, play as long as he feels like it, and then retire...say he sticks to his word and plays 5 more seasons total...subtract the 21 million and you are left with 15 over 3 years-or 5 million per year he would play...
Not saying this is likely, but the Celtics organization is excellent at communicating objectives to the players...if these guys want rings and there is a chance to add a major piece that makes it likely the team could compete for rings throughout, I could see Pierce giving up short-term money for the chance at long-term glory...