The way I look at it, I would have traded PP and KG at the end of their careers for a young PP straight up and I would have felt like we got the better end of the deal. Let's assume that JT is a young PP.
On top of that the Celts got Jaylen Brown, who is looking like a potential all star. If we put him in a fringe all-star category (like a Khris Middleton), we traded old PP and KG for young Paul Pierce and Khris Middleton. That seems like a win to me.
To be clear, I am not saying that the games of JT and JB translate into Middleton and Pierce, but the level of play is similar. Also, I am optimistic that JT and JB can be better than Middleton and Pierce.
Add in a couple of nice free agent signings and now we have a contender!