I see no reason to say that we can or cannot rebuild a certain way. Rebuilding is really going to be about acquiring assets and maintaining financial flexibility. Paul Pierce on the team or not really doesn't hinder that one way or another except this summer.
However, this summer doesn't matter. There's no one worth signing.
While he'll still be on the books come the center of 2013, if Danny really wants to move him, he'll likely be even more valuable then, as his skills will likely still be strong and he'll be an expiring contract.
Overall, I'd bring KG (and quite possibly Ray) back for 1-2 more years. During this time, I'd let the Big Three impart as much wisdom as possible on the young guys while staying very competitive (and possibly still contending). At the very least, this will develop a winning culture and give the young guys playoff experience. Come 2013 or 2014 (depending if KG gets 1 or 2 years), we then evaluate our situation.
Has Bradley become a star? Has Jeff Green further developed? Has anyone else we've drafted (Johnson, our two first rounders this year) become stars?
If Danny is confident he can build a new contender with this group, he uses the cap space from KG and PP to make a free agent push. If he thinks he can't contend with this group, he can look to make trades like he did to get the Big Three to begin with.