Wyc and the other owners cut off spending just after the '08 Championship. That's why they didn't re-sign Posey and Perkins.
Ainge couldn't go over the cap, so he changed his strategy to a long term drafting and accumulating draft picks. That hasn't been successful--it takes too long to grow your own stars, even if you're good at drafting. Besides, the league has come increasingly under control of stars in the James era--and in star clustering--now largely in the NY and LA markets.
Danny showed the way a long time ago clustering KG, PP and Allen--along with a bunch of veteran role players. The model now used by the Lakers and the Nets--and the Knicks and Clippers. But Danny got away from that model when Wyc cut off the money.
Having said that, the Celtics still have a strong cluster pair in Tatum and Brown, but need a 3rd star plus more veteran role players. They lost a chance for cap space for a 3rd star in the bad GH trade and signing/losing Fournier. Now they have to go over the cap to do it, and the ownership won't.