There must be a normal wear and tear cycle to be reached with spacecraft. Even gun barrels, a relatively simple device, have to be replaced after a certain amount of firing. I can't imagine the level of stress the frame of the space shuttles must take over the course of a mission. To keep reusing it for a 25 year period seems utterly negligant.
The shuttles were designed to last roughly 50 missions. This was Columbia's 28th, which is not the most that a shuttle has. Discovery (first flight 1984, 3 years after Columbia's) has 30, while Atlantis (first flight 1985) has 26. Even Endeavour (the replacement for Challenger; first flight 1992) already has 19 flights.
The shuttle's
airframe was originally certified for 100 flights; however, the
shuttles were intended to fly much more often. The Columbia was therefore like
an automobile that had only been driven 20K miles but was 30 years old. Their
equipment and 'running gear', if you will, is refurbished and upgraded periodically,
but the airframe itself is Original Equipment.
There's a reason why car makers say five years or 50,000 miles,