Isn't that what the offseason is all about?
The Spurs don't need a 'great backup point guard', IMO. They just need a serviceable, experienced, true PG. Not a guy that can bring the ball upcourt to Manu, but a guy that can run the offense even if Manu isn't available. Every time TP or Manu have gone down due to some sort of injury in the past couple of seasons, even when Hill was around, the same question pops up: Who's going to run the point? Hill sucked at it for the most part, RMJ sucked at it, and rightly so. They're not PGs.
And no, you don't need to trade Anderson for a backup PG. But having a serviceable PG allows you to move him if you need an extra sweetner to add to the Dice contract in order to upgrade in another area where the team does have a real need: the front court.
Let's not forget that a couple of relatively inexperienced backup PGs (Dragic and Vasquez) kick our team's asses the last couple of seasons during the playoffs. There's good talent out there, even at the backup spot.
You should strive to put together the best team you can possibly have, at least on paper. That's the bottom line. Every position is important. Especially for an aging group like the Spurs that no longer have superstar talent in the roster. We need more consistent and solid contribution from everybody else.