Code doesn't rust or corrode or accumulate bugs as it gets older. Obviously things can change around it, i.e. Dependencies and that can lead to some mandatory changes but really there are plenty of software that is old. Much older than 20 years and it's "ready" i.e. Has All the features it needs and it's still being built and run on modern computers and it works fantastically. My own software goes back +15 years now and over the years I've ported it from 32bit builds to 64bit builds and from Qt4 to Qt5. Other than that very few new features or changes in the past years since it has all the functionality that is needed. And it works great. What's wrong with that?

Changing code (or UI or UX or anything really) just for the sake of changing it is just nonsense busywork to me.