TP to the OP for broaching the subject in a logical manner and at least providing a scenario (albeit I don't particularly care for what we'd get back).
I'd like to see Pierce retire a Celtic, but some people in this thread act like Pierce making over a hundred million dollars to play here was some sort of charity act by Pierce. He got paid more here than he probably would have been paid anywhere else. IMO no player can be considered untradeable, but when you combine Pierce's current contract + the benefit of him retiring a Celtic + what we'd get back in a trade, I don't think trading him is worth it. We realistically wouldn't a lot back.
If the theme is to trade great players who still have some value in order to provide a transition to a quality future team, then you should start with Ray. He's the easiest to replace on our roster, has an amicable contract, is very durable, and was already highly sought after by contenders last off season. KG would be second, but I don't think we'd get a lot back for him.