He's making up for lost ground after wasting a whole season by failing to recognize the big three's Championship window closed (in 2012) and capitalizing on their peak value.
With their defeat in the 11/12 Eastern Conference Finals (second consecutive post-season defeat against the Miami Heat), what's beef between Ray Allen and Rajon Rondo → Doc Rivers → Danny Ainge (which inevitably forced Allen to leave in the offseason), Paul Pierce and Kevin Garnett both on the decline (showing they could no longer handle carrying a team between the two of them), ispo facto - that was the end of their Championship window (07-12).
Rajon Rondo's value was at its career-peak, and just like Pierce's and Garnett's would decline from there on out - the perfect opportunity to capitalize on what could have been fast-tracking a complete rebuild (saving them at least 2-3 seasons), with Allen's/Garnett's no-trade clauses as the only obstacle. Pierce/Garnett and most importantly Rondo could have netted Boston, a combination of the following through S&T or lottery picks (+ picks in future drafts):
'12 Draft: Anthony Davis, Michael Kidd-Gilchrist, Bradley Beal, Dion Waiters, Damien Lillard, Harrison Barnes, Terrence Ross, Andre Drummond.
'12 FA: Goran Dragic, Jamaal Crawford, Nicolas Batum, Ryan Anderson, Brook Lopez.
Following that: any lottery picks in the '13 and '14 Drafts and enough cap space for '13 and '14 FA.
Instead Ainge's imbued practice of empty loyalty wasted a whole season, already knowing their run was over, - watched the Celtics bow out in the first round, with KG/Pierce having their worst seasons since '07 and Rondo pushing his value into limbo - following his ACL injury. Only to trade KG/PP for three first round picks from the Nets: James Young (2014 pick), 2016 1st rounder and 2018 first rounder in the off-season.
Now that's a reasonable exchange, considering their value basically bottomed out, but they received the less favorable 2014 first-round pick, drafting a player whose max-projection is a 6th man/bench fill-out and two unprotected picks in 2016 and 2018. Waiting a further two seasons, which could turn out to be late first-rounders given Prokhorov/King could right Brooklyn's ship before those picks come into play or the fact that the East could be in the same dire state in 2016, they're still not bad enough to miss the playoffs.
Following that: Rondo's value now is basically Ricky Rubio with a invisible cloak that's rendered his flaws in purgatory to the basketball faux-cultured status, and if Ainge can't use that to his advantage to strong-arm a team into a top-heavy deal - he'll either lose Rondo for scraps or he'll eventually lose him for nothing, when he's a UFA.