When your best player, and future HOF bigman is on the rapid decline, and there is no budding superstar on the horizon (a la a David Robinson or another Tim Duncan), your championship window is likely closed.
Having a decent regular season record and being just good enough to get into the playlets is for mediocre, middle-of-the-pack teams. There are only 2 ways of acquiring the type of superstar-level talent that it takes to compete for championships - either via free agency or through the draft. And the Spurs are probably not going to get such a player free agency.
That said, the opportunity to get a young, developing and talented big is rare. If this kid is available at #29, the Spurs would be foolish to pass up the chance.