A smarter person could word this better, but different forms of long-form writing has differing expectations of quality. Though anyone can write anything they want, people are going to judge the same quality of writing differently based on whether it's fanfic, original fic, indie published, trad published, etc.
Fanfiction probably has the lowest expectation of quality. Readers are familiar with the the source material it's connected to, so that does some of the heavy lifting. This isn't to say that all fanfiction is low quality. I know there's fanfiction on par with classic literary fiction in terms of technical skill, and ultimately, fanfiction is free and done for fun, so it shouldn't be held to a specific standard anyway.
There was this oneshot fanfic that explored how the main cast reacted after a certain death. Everyone was written perfectly in character. Overall, it was a really touching piece about the ways grief can manifest and how it changes over time, even if it never goes away. It also had a lot of run-on sentences, which sometimes made it difficult to tell what was going on. I loved it anyway.
Had it been original, I might not have finished it.
Freely available original fiction is pretty close to fanfiction in terms of expectation with one big difference: The author is solely responsible for making the reader care. Character establishment, worldbuilding, setup & payoff, etc., these are all things the author has to provide on their own, and they have to do it well enough to make a reader stick around. So long as it's free, readers are willing to give some leeway, even if more work needs to be put in than fanfiction.
Once money is involved, standards get harsher. I've seen writers and readers alike sneer at the thought of indie published books under the assumption that they're not good enough to become traditionally published. It's a shame, because as I understand it, trad publishing picks up works based on whether they'd be popular, not necessarily good. Some real gems are independently published. That being said, the lower barrier of entry allows for more flaws to get through.
The next step up in terms of expectations is traditionally published genre fiction. Obviously, expectations are going to vary depending on the genre, and these expectations tend to fall in line with tropes. Genres require certain tropes. You can't have a romance novel without a couple, and you can't set historical fiction in the future. Science fiction and fantasy are harder to pin down, but if one of those stories are indistinguishable from a contemporary novel, it's probably going to disappoint readers. Stories of any genre can still find their own ways to be subversive within these tropes, but each genre has subgenres, and the smaller the niche, the more tropes it'll have to adhere to. Ultimately, the success of genre fiction stories depends on whether the readers find it entertaining.
Finally, there's literary fiction, which is hard to define but most people think of it as the written equivalent of high art. It's artsy and has capital-T Themes that it's expected to deliver on. It may not be as popular as genre fiction, but it's expected to be written by those with high technical skill and have something interesting to say.
I don't really have a conclusion to this. It's just something I thought about after hearing about a book that has the looks and acclaim of literary fiction but is written with tropes and a plot structure that rarely shows up outside of fanfiction. I haven't actually read the book, so I can't really comment on it. I just think it's interesting how these expectations can affect how a story is judged.
