Why the Blackburn Buccaneer could not serve indefinitely

Was 1999 an overly optimistic out-of-service date for the Blackburn Buccaneer?

Yes; the aircraft had already outlived their expected structural life by the late 1980s, and the airframes could not safely yield much more service.

Blackburn's expected airframe life was roughly fifteen to twenty years, and the surviving aircraft had already exceeded that range by the late 1980s. Their physical condition showed the accumulated cost of continued use. A replacement program might slip, but that did not create unlimited structural life in the existing fleet.