I agree-- if Roger Mason continues to improve, he will be an upgrade over an older Finley. His shooting percentages are better than Finley's, so if he can average 12 ppg and play good defense, the starting lineup will be quite a bit improved. The Spurs don't need a ton of scoring from the rookies--they mostly need good D, smart play, limited fouls & turnovers. If Mahinmi, Hill, Gist, and Tolliver (assuming they're all on the opening day roster) provide 12-15 ppg combined, that will replace what Horry & Barry scored. If I had to bet right now, I'd guess they're going to bring back Finley for one more year.
I really think the defense and rebounding will be improved next year, meaning any small improvement in scoring will have even greater than usual benefits.