1) Where do you think original fiction ends and where does fanfiction begin?
I mean
- Dante's Inferno is literally a self insert fic of the Bible with cameos from ancient Greece, a few Popes, and the poet Virgil
- The Aeneid is a fix-it fic of the Trojan war, told from the perspective of Trojan refugees
- Superman also took inspiration from the Bible
- Rick Riordan's books are based on various mythologies (Percy is literally the reincarnation of different Greek heroes, and imho the only reason it was not made explicit in the books (that I've read) was because Disney was lurking in the shadows waiting to sue Riordan over Hercules)
- Disney made its fortune by ripping off Grimm and Anderson's tales -- and they had no qualms about changing their source material, the themes or even the moral of the original stories; that doesn't mean Disney's animated movies are bad or even have a bad moral
- O Brother is an (admittedly somewhat loose) AU of The Odyssey
And what about all the Sherlock Holmes movie/play adaptations? Elementary, Sherlock and House MD, all very successful TV shows, are all modern day!AUs (and if you think about it most detective series have no shame about copying the Holmes/Watson dynamic). Hell, Star Trek TOS' Spock and Kirk take a lot of inspiration from Holmes and Watson respectively.
There are people who write new stories about Sherlock Holmes every year. These stories are sold in stores, and no one seems to have a problem with that, despite the fact that these stories are essentially fanfiction.
Sir Arthur Conan Doyle himself took a lot of inspiration from one of Poe's short stories (especially his duo of protagonists in
Murder of the Rue Morgue) to create his iconic characters.
The biggest difference between these shows and books, and fanfiction, is that the people who made the former were paid for their work.
2) Fanfiction is like baby-wheels on a bike for writers. You don't expect a 2 year old to learn how to ride a bike without them, do you? Sometimes it can be less intimidating to reuse characters and a setting that are "already there" and that you know than to try and create everything from scratch. Kinda like modding video games actually -- Team Fortress was originally a mod for another game, wasn't it?
3) Your Oscar Wilde quote seems to kinda go against you own point? because
a) not all people are born great, some have to work for it, and
b) this was the guy who was the literal embodiment of "Fuck the police, I do what I want" -- he put barely veiled gay allusions in his works at a time where being accused of gay letters could get you sent to prison; he did go to prison because he couldn't keep from mouthing off at his own trial for "gross indecency" (ie being gay). If he was alive today you can bet he would have a Livejournal, a ff.net and a AO3 accounts and would be writing gay erotica fanfiction of his favourite shows like there's no tomorrow
4) Not everyone enjoys reading fanfiction. That doesn't give those who don't enjoy reading fanfiction the right to say that fanfiction is inferior to "traditional' fiction, or useless. I don't enjoy reading biographies, but I'm not going around saying that biography is inferior to other types of fiction, or useless.
5) Fanfiction writers (and people who make fanwork in general) don't get paid for their work. Fanfiction is free, easily available entertainment. It doesn't take money away from the big companies who own the copyrights to the original works these fanfics are based on. The fandom who produces fanwork keeps the interest of the fandom into the original work alive AND
promotes the original work. So I think both the author and the company can tolerate a few awful fics that don't cost them anything anyway, because it's good for them in the long term
6) You seem to labor under the misconception that all fanfiction is poorly written, badly plotted, and betrays the original work's themes or message. And that can be true: there are a lot of crap fanfics, sure, but
- there are also great fics out there -- I have, in all honesty, read fanfics that were better written than some published books. I have read fanfics that were not as well written but that had a much more interesting plot (to me) than most published books. (although I agree that there are crappy fanfics out there, but then again there are also loads of crappy books)
- not all published books are good -- Twilight comes to mind -_-
There are also a lot of fanfics who don't take the original work's message or theme, but that doesn't make them bad, just different from the original work. Again, to use a video game modding analogy: It's not very different from those Skyrim mods that turn dragons into MLPs or spiders into bears, imho. The experience intended by the devs is ruined, but the players who use these kinds of mods still have fun. And in most cases it's not as bad as, say,
turning Captain America into a literal Nazi.
7) People write fanfiction for a lot of reasons:
They love a character, or the world they are in, and they want to see more of it.
Or they give them a different ethnicity/gender/sexual orientation because they are desperate for representation in a world that caters most to cis straight white men.
Or they hate a particular character and want to "bash" them.
Or they want to begin writing, and using already established characters and settings is easier on beginners (and more motivating) than creating everything from scratch
Or they love (a) character(s), and want to see how they would fare in a different universe/setting/etc... (Alternate Universe or AU fics for short)
Or they want to get off to 2 or more characters getting off because they think it's hot
Or a number of other reasons.
Or a combination of the above.
Okay, rant is over. Sorry for the wall of text.