Ronde Barber is old. Really old. At 36 he's the second-oldest cornerback in the NFL, behind only Al Harris. While Ronde Barber is still playing at a high level, he's not the same player he used to be. He's lost some speed, which gets him into trouble when he tries to jump routes and can't recover. In addition, he has led the league in missed tackles over the past three years with 44. Part of that is his size, but a much bigger part of it is the decline of his physical talents.
In some ways, Ronde Barber is already declining, but he's still a very useful player. His instincts and intelligence allow him to compensate for his declining physical skills, and his abilities as a slot player should allow him to be in the lineup even when he can't be a cornerback on the outside anymore. The fact that the Bucs will protect him with zone schemes and safety help should allow him to continue to contribute for a while longer.
But, at some point the best cornerback in Bucs history will not be able to compensate for his limitations anymore. The Bucs have planned for that day for years now, drafting cornerbacks in almost every draft. They now have a young nucleus of talent at the position with Aqib Talib, Myron Lewis and E.J. Biggers, though it remains to be seen whether Talib remains with the team for a long time. At some point Ronde will not be able to play anymore, and the Bucs need to be ready when that happens.