Let's face it, even the Chief was able to put in over 9 mpg at the age of 43 for the Bulls.
If the Chief could do that near his mid-40s, then Pierce can easily do 5 mpg, until he can't run anymore.
So that's it, once the next season is over with the Wizards, Pierce should rejoin the C's at Vet's minimum and keep playing for the rest of his natural life. When the rebuilding is over, he'll be winning more championships but with everyone else around him, hoisting much of the responsibilities. His job will be to come off the bench, knock down a couple of shots, like MacAdoo did for the Lakers, and go back to rest. I believe that even Hakeem can do that today, despite being in his 50s.