IMO, there is a clear and obvious upgrade to this role who happens to be a FA this year, but will likely cost a little more than Keldon, and that is Malik Monk. He's already obviously quite proven in this role (5th and then 2nd in 6OTY voting in the last two seasons), and it a more fluid scorer than Johnson. Keldon, despite putting up scoring numbers, seems to labor to get his, whereas Monk's seem to come more naturally in the flow of the game.
Personally, I like a Cam Thomas-like player as a potential ideal 6th man. With that said, I don't think Cam is worth going and expending considerable assets to acquire (but would be fun in a Bridges-Thomas-Cam Johnson mega deal to completely shake up both rosters).
I just don't know how well Keldon really fits in the style the Spurs want to play. He was part of a very productive closing lineup for the Spurs, but there were times when paired with Wemby and Vassell that seemed to really disrupt the flow of the offense, and he's of course not very useful on defense either.
The Spurs seem hopeful that Branham could fill that 6th man scorer role, but the jury is still out. If the Spurs draft Dillingham, he could be that 6th man scorer in his rookie year before taking over the starting gig next year. If they take Holland, I also see him as a potential contender for that role (and a direct replacement for Keldon's minutes).
In the end, seems like the desire to move Keldon is less about Keldon the player or person and more about his fit and the desire to capitalize on his value while he still has some (his contract helps in that regard).