Tim may or may not be done in 3 years, but he will certainly be past his peak. So will Manu.
If the Spurs had a track record of making the big Free Agent scores, maybe this would make sense, but they don't.
Parker could become a dominating point guard. I don't think so, but for the sake of argument let's say he becomes the best point guard in the game.
Dominating point guards don't win you championships. Not unless Mark Cuban still has some more left to his improvement plan.
The Spurs may be a good, entertaining team with Tony Parker as a first-team All-NBA, I just don't see them winning les past this three-year window.