Why not? Because the Spurs don't need a SF that can "still put up a good game". They need a SF that has the ability to provide a steady stream of offensive production on a consistent basis. At the same time, helping to defend and shutdown the player that he's guarding. That aint Finley.
Why not? Because he's a liability on defense, doesn't block out, can't create his own shot and can't get break down a defense. While he can be a serviceable shooter on occasion, Finley cannot be counted on for consistent offense on a game-to-game basis. This is an old story, but Finley is and has been a one-trick pony. His defects override any value that he brings.
Why not? Because Finley is to Pop, as candy is to Lamar Odom. If Finley is on the roster, Pop will not be able to resist the tempation of overplaying him. If the Spurs are able to successfully trade for or draft a player(s), who can earn minutes at this position, having Finley on the roster will be counterproductive and will stunt the growth and development of said player(s).
Why not? Because it's time to turn the frickin' page already. The SF position needs a complete makeover. The Spurs badly need a talent and youth injection - especially at that position. They sorely need more players, who have a better assortment of all-around skills. Investing another year in Finley gets them no closer to that goal.
Why not? Because it's 2009, not 2007. For the past 2 seasons, we've seen this movie play out and it aint good. The older players on this team have gotten older and the talent level has regressed. Meanwhile, many NBA teams have gained ground, while others have passed them by. Look at the final four teams in the NBA playoffs and find one that starts a 37 year-old SF. Don't stop there. Scan the rosters of every team in the Western Conference and see if you can find one team where the SF matchup favors the Spurs, with Finley as the starter.
With all that in mind and after watching the Spurs last 2 playoff series, versus the Fakers in '08 and the Mavs in '09, isn't enough to convince you that the time has come to move on past Finley - then nothing will ever convince you. I strongly hope that Finley either decides to retire or that the Spurs have the "difficult" conversation with him and convince him to do so. It's time to move on.