Pierce's game would have to decline a LOT in the next two years for him to be only an MLE-level player (implied by the $6M projected salary).
I tend to think of Pierce in two years as being about equivalent to Ray Allen today. Ray's a little older, got a little more wear and tear, but both long-time All-Star wings. What kind of extension is Ray Allen thinking about, now? $10M/year? $12M? Something in that ballpark?
Short of a significant injury or a 'Toine/Steve Francis-like dropoff in production, I don't see there being a huge risk for Paul playing out his contract.
In reality, what'll happen is they'll negotiate an extension next summer. Paul's the face of the franchise, they aren't going to let him walk. They want him to represent this whole era of Celtics basketball. In 2020, when Paul's in the HOF and we're celebrating Banner 23, they want Paul in the building to shake the new star's hand. They won't disrespect him by asking him to opt out, and Paul won't leave money on the table.