Hmm, that is a tricky question and I think you may get a variety of responses. I know some people who have long left fandom may have wanted their fic to be lost to obscurity (so to speak), so if they decided to not update their fics to a popular archive, if it were me I might be inclined to not move them to AO3. I know it may be sad to see the stories go completely, but not knowing the person's thoughts I guess I might save them for my own personal enjoyment to read again but that would be about it.