Con No. 3: Rudy's age and decline
At age 34, Rudy can still contribute to the team, though he has shown clear signs of decline. He is not nearly as quick or athletic as he once was, which has affected his play on both sides of the ball.
Furthermore, Gay has become more reliant on shooting threes and less reliant on attacking the basket. While Rudy has evolved into a good 3-point shooter, if he regressed, it would have a significant effect on his scoring efficiency.
Gay can still score and act as a mentor to younger players next season. That said, given his age,
Rudy simply doesn't fit with the Spurs young roster long-term. Where he to return, it would certainly be on a one-year deal or else the Spurs would risk paying him as he continues to decline.
Con No. 4: Financial Cost
Rudy Gay could receive interest from several contending teams, possibly driving up the cost for the Spurs to re-sign him. While he's likely to receive far less than the $14.5 million he made this season, San Antonio may have to overpay to get him to stay.
San Antonio does have plenty of cap space, though it seems unlikely that they would overpay to re-sign him. Gay, on the other hand, came to the Spurs to compete for a championship. It's doubtful that he'd want to return to a team that is unlikely to make the playoffs without financial incentive.
Ultimately, Spurs appears likely to let Rudy leave and opt to pursue younger, lower-cost, power-forward options in free agency.