When Tim Duncan retires, so does Pop. And, then even with Tony Parker in his prime, there will be quite a few down years for the Spurs. Every great franchise goes through this. Without Tim Duncan and Manu and most likely Pop, why would the very top free agents want to join the Spurs? Great organization, but without Duncan and Manu and Pop, it's a tougher sell.
Maybe guys like Boris Diaw or Johan Petro or Nicolas Batum are superstars by then and can be lured by Parker. Other than that, the superstar big men like Greg Oden and Dwight Howard and Chris Bosh are not going to be easy catches for a team like the Spurs. Now, Lakers or Knicks, at least they'll have a shot because of the cities. But, best bet is a second tier star.