Might as well let him ride out the contract. There really is no big benefit to trading him. Not gonna get great value for him.
Well, the benefit to trading him is 2 things.
First, salary filler. If you want to get a younger, impact player, chances are you will need a way to match their contract, and an expiring contract like Pierce's can be very valuable for that.
Second, you would get SOME value for him. A contender would love to give up an expiring contract and a young player or pick for him, I would imagine.
You are right that you are not going to get equal value for what Pierce does for this team. However, if you are commited to moving on, he would be a valuable piece to move.