Friendly reminder that fan fiction completed A Song of Ice and Fire (basis of HBO series Game of Thrones) after author George R. R. Martin seemingly abandoned it. Martin would go on to release one more book, but dropped the series after that, instead working on prequels and spinoffs for TV with HBO. The fanfic most commonly credited for completing the series is called "The North Remembers" and, as you can guess from the title, features a Stark victory, which is in line with how the series began. I'd say that's as much a victory for fanfics as the Fifty Shades series.

As for commercial writers embracing fan fiction, a collection of fan fiction based on Stephen King's The Stand was just published last year. There are some commercial writers in there, but the common thread is that they are all writing fan fiction that is set in King's post-apocalyptic (by way of a superflu) world.

Some would argue it's not fan fiction if it's published or if it's endorsed by the original author, but I don't think that matters. I think anything based on existing work is kind of fan fiction. This is a slippery slope though. The isekai genre (popular in Japan, but we use it here; it means "other world," as in you're trapped there) is based on Alice's Adventures in Wonderland, and one of the most popular isekai, Sword Art Online, even pays tribute to Alice directly in the third and fourth seasons by naming and designing a character modeled after her. But Reki Kawahara didn't write Alice in Wonderland fan fiction (though some say Sword Art Online started out as fan fiction of .hack//SIGN, an older and similar anime), he just paid tribute to the character. Flattery, not imitation. And in Kawahara's stories, Alice isn't the one trapped. His main character is instead trapped in Alice's world, which, as it turns out (this is not a spoiler, it's known from the start) is a virtual world. She's an AI but she doesn't know it. It's more accurate to say they're copied souls, but what it amounts to is basically the same. They are simulated people.