In my experience, I have rarely seen anything which is feature complete. Either UI needs revamp because it's stale and not in current trends (like fashion industry) or the the backend starts to crumble with the weight of new ad-hoc features, increased number of users etc.

With agile, practically, many times development is just in time with little regard to long term requirements or the evolution of software so it's all layers of layers of things that work. There's rarely a time for fundamental restructuring of the codebase to meet the new requirement correctly.

Once the cracks begin to show, we start thinking of software rewrite.

For a construction of buildings, the redevelopment is about once 40 years, for software, it's about 5-7 years.