Patty's a much more complete player than Forbes or Murray right now. Bryn is not a dynamic shooter at this point, and he's not a play-maker. He competes on D, but he's not noticeably better than Mills on that end either. He wouldn't be the back-up PG, which is why Murray still factors into this. We already saw how exploitable Murray is right now. Until he fixes those issues, he's going to be a worse option than Mills, especially if Patty remains aggressive and effective in his shooting.
In a scenario where the Spurs were looking at cap space, money-balling Mills into Bryn might make sense. But it doesn't in most likely scenarios.