Derek Fisher may not rack up lots of assists, but he has been a starting-caliber point guard his entire career. He wasn't a shooting guard in college, he was a point guard who set team records for assists. Without Parker, the Spurs don't have anyone who can do it consistently on the entire roster. George Hill is not a point guard. The disaster of having him run the offense in game one of the playoffs should be enough evidence for anyone. Hoping for a guy who airballed a critical three pointer from his spot to be as clutch as Fish is, to put it kindly, a pipe-dream, and is only exceeded by thinking he can be transformed into an adequate point guard on a team without one.
The Spurs have already suffered enough from not having a rotation that allows people to play at their natural positions without hamstringing them further. If you want to try to build the roster through trades, you have to start with a position at which you have some depth, which is shooting guard, and you have to move a player with some value, which is Hill.