San Antonio isn't going to rebuild while Tim Duncan is still on the team. They won't do that to him. They are going to try to maximize his remaining few years. When Duncan goes, rebuilding will be back on the table ... but not until then.
A salary dump makes no sense for the San Antonio Spurs.
it does if the Spurs:
(a) are short on money (the proposed trade, coupled with not picking up Miller's option, would save the Spurs $20M this upcoming season alone);
(b) they figure they can't win a championship with OKC looking better, Memphis looking better, Lakers being the Lakers (and that's just the West);
(c) they feel they won't drop off *that much* by making the proposed trade, so their fan base won't get too ticked off.
Also, Duncan has only one year left on his contract. If the plan is to rebuild next summer (i.e. after Timmy retires?) then why not start a season early, given (a) - (c)?)
Now, all that said, I don't necessarily like the trade, I'm just trying to think through reasons why it might not be as outlandish as people think.