i do agree with the fact that writing fanfiction isn't writing on "easy-mode" and definitely understand why people might find that characterization insulting or patronizing. i definitely used the skills i learned writing fanfiction to figure out how to write original fiction; but i will write fanfiction forever because it as a genre offers intellectual and aesthetic pleasures that original fiction can't offer, much of which is covered in this post.
however, i do think this framing kind of ignores the fact that coming up with your own characters, world, etc. that is necessary to write original fiction engages a different skillset, based upon my own experiences writing both. this could have been a "me" issue because, in part, my issue with original writing was that i expected it to work like fanfiction. in the sense where writing interactions between characters felt more like how it felt when i was in the hyperfixation mines with my two blorbos and things just kind of progressed naturally. writing is often to me about making lots of choices that lead to more choices which then lead to the feeling that there is an internal logic or mechanism to what you're writing that you don't have to pause to make each decision and then you're in the beautiful flow state just barfing up words until you can't anymore.
in fanfiction, there are more boundaries around the decisions you can make. that doesn't mean that you aren't still making important decisions; but that -- if you are working with an existing character, some aspects of their characterization have already been decided for you. even with the transformative nature of fanfiction, there are usually a couple of attributes of characterization that become tenants or even memes of the way that character is depicted in fic. and you can obviously play with that and those expectations but, by doing so, you are again building off of choices that have already been made in order to make your own. these boundaries often lead to what i think makes fic so special -- being surprised by a story about a work you hold dear even though you have read so many of them already. however, for me, it often makes getting into the flow state easier because i am not coming up with the internal logic of the story from scratch.
and, again, this might be "me" problems. but when i started trying to plot out original works, i always underestimated how much thinking needed to be done about the characters before i could start writing. not because that when i write fanfiction, i am not thinking about the characters, i am a lot. but it is really hard just coming up with a character who isn't tethered to anything. of course, the way to fix that is to start adding new components around them such as a setting, others characters, etc. for the novel project i am currently working on, i have had some version of these characters in made for like five years. only in the past two have i figured out the correct setting and set of circumstances to put them in where i feel like i am able to finally get into the flow state that i used to just associate with writing fics.
I do think writing fic for a long time has made me a way stronger writer than some of my peers who never did it. writing dialogue and interactions between characters always came easier to me. i am so thankful that i have writing fic as part of my genealogy of being a writer because of that. but also like, it isn't wrong to say that writing original fiction takes other skills that are difficult to cultivate within the bounds of fanfiction, in part because of what makes fanfiction so great.
anyway, i do not disagree with anything OP said. but i do think it is important to talk about the different skills that go into writing both fanfiction and original fiction. and that coming up with your own ideas isn't necessarily HARDER than writing fanfiction, but engages a set of skills that you might not have developed yet if you've only written fanfiction up until this point.
