Some may be past their prime, but they are still exceptional players. Garnett may have his best years behind him, but he still has some great years ahead, same with Pierce and Allen. Wallace is only playing the bench anyways, and Det wouldn't have let him go if he was still in his prime.
But they also still have great young talent. Rondo is only slightly inconsistent, but he gets better every game, and is the best all around PG other than Chris Paul in my opinion. Plus Powe is great, he just doesn't have the opportunities, same with Big Baby. Perkins is still really young too, and he's a solid starter. They still have a couple years to make really good runs, and I still think they're better than LA.