Fanfic serves a lot different purposes. It fills in blanks and gaps in the official narrative. It fills in the gaps left by end-of-season hiatuses (and the mini-hiatuses). It brings a show's characters under microscopes to examine motivations and other aspects of the inner life. It allows writers to interact with the characters (and the readers get to go along for the ride). It allows both writers and readers to explore what happens when the characters are put into different situations than what appears on screen. It does allow for wish-fulfillment. There are as many reasons for fic as there are writers. Some of them are pretty darn good reasons. Fanfic is part of what makes a fandom more than the sum of its parts. There is no reason why any of that should be less appropriate after a certain age. If you are not convinced, find some Doctor Who or Star Trek fans who watched from the beginning and ask them what they think.

There's a quote knocking around tumblr that fic is culture's response to our mythology being owned by corporations.

Yeah, that's from Henry Jenkins' original book.

Can't stop the signal.

fic is culture's response to our mythology being owned by corporations

So is it not the same sort of fic if it predates corporations? Because I've beens saying fic is a response to consuming a story, not to the ownership of the story, and therefore it goes back many a year.

I agree, fic is a natural human response. But, in non-copyright situations, there's a legitimacy that post-corporate fic doesn't have in the broader culture. I don't completely agree with the sentiment, but I think it's near enough to be useful as a starting point.

Because I've beens saying fic is a response to consuming a story

I agree with this. I mean, in the way I mean fic where it is freely distributed though possibly within known communities, the legal status of the original story or characters or people isn't the point. It's a response to or continuation of or different look at the story.

Fic that undergoes another transformation in an attempt to sell the work and probably try to make money off of it is a whole different story. But it's no longer what I'd call fic at that point, so it's irrelevant to me when defining fic.

So is it not the same sort of fic if it predates corporations? Because I've beens saying fic is a response to consuming a story, not to the ownership of the story, and therefore it goes back many a year.

I can see how this is a reasonable way of defining it, but how are we going to convince people that what Shakespeare wrote was fanfic?

I consider the onus on their part. I can't, apparently, resist the argument, so I just list stories, and ask them to tell me why it doesn't count. I don't usually go to Shakespeare, but the premise of any re-used character is good.

Folk tales and folk songs that have as many variations as there are story-tellers and singers - I'm not sure that it is fair to count those as fan fic. And yet I feel sure that the variations come from the same urges that fic emanates from.