I'm not talking about technology, I'm talking about the structure itself. For example, modern aircraft are certified for specified service life or service cycles, not in expectations of better technology, but because metals fatique and wear out. Eventually, microcracks and granular imperfections will cause flaws that will cause the structure to essentially grow old and break. If you watched BSG, the same thing happened to Galactica; its wear was a result of age, compounded by a tough life.
Terok Nor is old and lived a hard life. I imagine its structure is riddled with micro fractures. You can only maintain and refurbish such structure defects so much, take it from an aerospace engineer.