Also when did it become fan fics Its called doujins, i refuse to call it fan fics anymore.
And don't take my opinion seriously since i have only read a doujin of bunny girl senpai.
Fan Fiction has been around forever. A lot of Silver and Bronze age comic books began as fan fiction of Gold and Silver age comic books; heck some of the first comic books were, essentially, fan fiction of pulp novels and radio plays (The Lone Ranger, The Green Hornet, The Shadow).
More recently, the Fifty Shades of Gray series began (literally, as the author has admitted several times) as fan fiction of the Twilight series (but, after editorial and legal-mandated revisions, wound up its own unique piece of slime that shares only a pathetic female lead/MC and a grimdark handsome male lead).
And the entire Flashman Papers series of books was, pretty much, fan fiction based on a minor character from
Tom Brown's School Days.
Fan Fiction has been around forever. Heck, the oldest piece of fan fiction I've read was
Silverlock, which was written during World War II, and the Harold Shea novels of L. Sprague deKamp and others are best described as fan fiction with modern characters inserted directly (on isekai needed) into mythological or fictional worlds.