I sometimes work on them for my own satisfaction, not as a resale item, but the reality is someday all our collections will be distributed to new homes (or the trash bin). I think a lot of it is keeping originality. If you use original parts, it is keeping with the collectability. As these models are often 80 plus years old, glues weaken and parts fall off. Replacing the odd gun barrel or mast or touch up a spot is par for course.
If I do extensive refit - making new structures, repainting -etc. I use a burner to mark underneath that it is refurbished. Labeled ones tend to get left alone more than unlabeled (RN-ID) types. Some "new parts" I have done for people are mounted non-permanently, such as with "Museum Wax".