I think it's more unlikely that he doesn't come back at some point.
I'd bet retiring as a Celtic would mean a lot to Pierce. How could he have a more perfect end to his career?
The way Pierce is playing, I could see him in the league for another 4 years. His defense is still really solid, as long as the shot keeps falling, why not? If Nash could do it with negative defense, what's stopping Paul?
Porter should probably be starting in a year or two, and I couldn't really see Pierce coming off the bench.
If Boston improves next season and moves over .500 I don't see why he doesn't consider it. We don't have any definite answers at SF. I think he'd love to play with hard-nosed players like IT, Bradley, Smart, Sully, Crowder.