Operational lifetime...
Additionally, the Columbia is the heavy duty bird. She's stronger than the others, designed to take more punishment (which of course no one would intentionally inflict on her). She would be operated as an OPERATIONAL flight vehicle, not a developmental one. Mods could of course be suggested by the ground crew, but the basic intent is to fly as is. That's why all existing documentation associated with the Columbia would be included in the initial $2.0Bn cost, so that it can be analysed and a strategy developed for its operational use. She's been fine for over twenty-five flights, she should be fine for another thirty-five or more.
My car tyres have done 25,000 miles. They should be good for another 35,000...