I’d likely still delete them after I complete the section the route covers or I plan another route that goes over most of the same area.
Specifically for myself, I don’t see a need for an inactive list since once I run it, I likely won’t run that area again and thus don’t need to refer to route again. This is unique for when trying to complete all city streets though. If I were to use CS route builder to create a route that doesn’t have the objective of completing streets, I may keep a route around longer. Ex: Create a route that follows race course I plan to do in future or follows some nice trails that I want to frequent in future.