Code rots, even the best code rots, because the people and cultural environment that the code was written under change and fade away. Refreshing it occasionally, even if it isn’t improved in a significant way, is one way to deal with that rot.

Even if the product/library/framework never changed much, rewrites would still be necessary to keep it going as new generations of programmers shuffle through. Otherwise we wind up with a Verner Vinge dystopian culture of software archeology.