They were only designed to last 20 years, the remaining ships had done many more flights than they ever were intended to. More disasters were going to happen. The big problem isn’t their retirement but that 5 presidents in a row failed to greenlight their replacement.
The shuttles were designed for 100 missions each. The high-time shuttle is Discovery, with 39 flights.
There were some aging issues, specifically the discovery of the frayed wire bundles. But the spaceframes themselves were in good shape and could have gone to at least the 100 mission mark and probably a good ways beyond. Just with a LOT more intensive and expensive maintenance.
Yeah, but check out the F-35...