It's true that rewriting everything is not always the right answer and that fixing existing systems is underrated. However, there are very few instances in history of systems being indefinitely fixed and upgraded. Most societies were not gradually improved and reformed forever: eventually they fell and were replaced with new societies. The same is true of technology and companies: most technology is eventually completely replaced with newer, better technology that is inspired by earlier technology but that is still new. The cool thing about DeFi is that it has built-in ways of upgrading itself. In traditional systems, upgrades are more centralized and have a single point of failure: once that point of failure (eg the developer/maintainer) stops upgrading, the system dies.