Assuming there's no market for Pierce, I'd keep it together. Have him opt out, and give him a 3 year $30m deal. Move Lee for a backup big.
Get lucky in the draft.
Look at the teams still in the playoffs. West was picked 18th, Hibbert 17th, Hill 26th, Stevenson 40th. Shumpert was picked 17th, JR Smith 18th. The Bulls have Butler, picked 30, and Gibson, 26th, strengthening their rotation. The Warriors have three rookie draft picks playing serious minutes, two of them picked 30, Ezeli, and 35th, Green. mark Jackson calls all three of them grown men, and he's right.
If the Cs had the benefit of a full year of Sullinger, and picked either Draymond Green or Festus Ezeli with their other first round pick, they'd still be playing right now. DA needs to nail it with the 18th pick, getting a guy that fits for this team and can step in right away, a la Sully.
KG was one of the best bits in the playoffs. Pierce struggled because he's not a point forward who can conduct offense for 40 mpg anymore. Put a top PG next to him to run the team, and Pierce is still one of the top 20% at his position. Green has shown that he can be an efficient 20 ppg scorer. Bass is a capable bench PF, who has the added benefit of covering power SFs/fake PFs like Melo. Sullinger, if healthy, gives the team the rebounder and paint presence they need, despite being a bit undersized.
If the Cs can snag a couple of ring chasers for the veteran minimum, they absolutely could make another run with this squad. The alternative, cutting loose Pierce and forcing KG and probably Doc into retirement, sends the Cs into lotto hell.
I was in favor of moving on at the trade deadline, if DA could have gotten something like Jordan and Bledsoe for KG and a young wing like Harrison Barnes and scraps for Pierce. With Westbrook's injury, the West opened up, and the Clips would have had a great chance of going to the Finals with KG this year, and would have had ring chasers lining up to got here this summer for the vet-min. The Warriors would be better poised to Knock off the Spurs, and would have had a crunch time vet to settle them down in close games. Now I don't favor the rebuild because Ainge probably can't get much back for either PP or KG, and they both are top six guys at their positions.