They need another all-star -- Tim Duncan. This team goes as far as Tim Duncan takes them, period. Without Tim, there are no les and there are many, many fewer wins. They might be compe ive for a playoff birth without Tim (though I doubt it), but they'd never compete for a le. What the Spurs have works beautifully with Tim, but it all works because of Tim.
As for adding stars for the sake of adding stars -- it seems to me that both Portland and Los Angeles have tried the "many stars, fewer role players" strategy, but neither won a le that way. Can't say I'm inclined to believe that such a strategy is successful in the end.