It's never too late to add a chainstay protector of CF frames. So far you've only seen cosmetic damage, but that can change.

FWIW - chainstay protectors fail to protect the most critical place, which is right by the chainring. Chain suck or something similar can jam the chain up against the stay with the gap being too narrow to let it pass. This can cause serious local gouging, which a simple plastic film cannot prevent. It's a pet peeve, but IMO a ballistic plate, ie. shaped stainless steel, should be bonded to chainstays at this very critical location, to ensure the chain cannot cut into the stay.