In the summer of 2012, Danny Ainge was faced with a tough decision: blow up a team that came within a game of another trip to the NBA finals or keep it together for a puncher's shot at another title.
I think Ainge's Plan A was to compete for two years and for 2014-2015 to be a bridge year, with an eye towards clearly cap space and getting a good pick in that draft. Unlike the 2014 draft, 2015 was projected even then to have some potential franchise centers. Some even said it was better than 2014. (More are probably saying that now.)
Look at the contract situation at that point. Pierce expires in 2014. Garnett is partly guaranteed for 2014-2015 and was seen as possibly retiring. Rondo expires in 2015.
Look at the free agency deals. Ray Allen was offered a two-year deal that would have expired in the summer of 2014. Bass was re-signed to a contract that expires in 2015. Jason Terry was brought in on a deal that expires in 2015. Jeff Green's 2015-2016 player option makes it function like a tradeable expiring contract in the summer of 2015. Courtney Lee's deal would also be an expiring contract that summer.
The contracts look very much set up to potentially be rid of Garnett/Pierce/Allen after 2014 and to create cap space in 2015. The length of deals given to the LGBT bloc was probably a function of Ainge seeing the 2015 free agent class as much more desirable than the 2014 free agent class, rather than a stupid overpay bidding against himself.
Ainge left himself for several seasons in a row where he could have expiring contracts to work with: Pierce in 2013, Garnett's half-guaranteed final year when he might retire and Bass+Terry in 2014, Green+Lee in 2015. These are the sort of salary slots you need to swing a trade for a big-contract star.
But then Rondo got injured. With Rondo likely to still be less than 100% during the 2014 playoffs, the Celtics don't even have that long-shot puncher's chance anymore. If Rondo was going to be back by the start of the season, maybe Ainge finds a way to do a sign-and-trade for Paul Millsap, gets a veteran backup point guard like Beno Udrih, and brings in a solid center to help preserve KG, but the extent of his injury kills that idea.
Maybe Doc Rivers could stomach a quick one-year bridge between windows and would have stayed through a bad 2014-2015 season knowing that he would have the cap space to lure in some great free agents who would want to play for him in 2015, but wasn't willing to sit through two tank-y seasons. His leaving put a huge dent in the idea of Plan A. Without Doc, it becomes harder to bring in free agents, so that cap space in 2015 looks less useful.
And, so we're on Plan B. Blow up the roster. Get under the luxury tax. Stockpile assets. Wait and see how other teams react to the new CBA.