It is very simple, and Ainge knows because it happened to him, the Celtics either do everything they can to give Garnett and Pierce a title chance this season or they trade them away so the future is easier.
Ainge departed so the Celtics could get some help behind Bird, McHale and Parish. Pinckney and Kleine helped a little bit. This time, trading away Rondo for Josh Smith or Marcin Gortat could help a lot, especially if it is the last Garnett season.
The other part that the Celtics know is that old superstars have trade value, and can acquire parts for the future. After Bird won his titles, he could have been sent for Chuck Person and bigs Herb Williams and Steve Stipanovich. Schrempf and Sam Perkins were offered for McHale.
It is all about money, and Garnett retiring and Pierce being bought-out wouldn't free enough money under the cap to do anything great, but trading them now would meaning getting plenty of value back. If first round picks, young prospects on small contracts like E Bledsoe and some expring contracts, which could save money or be traded for other players at the deadline, like L Odom, arrive it could help the rebuild which will happen now or in one or two years.
The better thing to do is get Garnett, Pierce some extra help now. Rondo won't win anything in Boston when Garnett is gone so send him away for some immediate help. The Celtics can win this year, everyone can retire happy, then they can have a bad year next season and get a high draft pick in a much better draft than the 2013 one will be.
Another part of this, the Celtics could have traded away Ray Allen for a first round pick and OJ Mayo, instead they kept him and he missed shots and then left for less money and the Celtics got nothing for him.
Ainge should do something at the trade deadline, either everything to win right now or a total rebuild before he loses his trade assets to retirement, or possibly free agency.