While your suggestion might be what's the best move for the Spurs, I put a near zero possibility of SAC agreeing to that.
In reality, Houston is likely the better trading partner for SAC for Fox anyway. They can return picks and more young talent (Spurs fans don't want to give up any young talent, which is going to be a problem). SAC should probably also look for a total tear down by moving Sabonis and DeRozan as well, but I doubt they'll do that - so they'll attempt a soft reconfiguring as opposed to a total tear down, and it will probably just mean another decade of mediocre to poor results (hopefully through 20131)