Fanfic is subject to the Gauss curve like everything else.
The Lord of the Rings is sorta-kinda fanfic to Beowulf, the Witcher is fanfic to Slavic and Celtic mythology. Neither of these two authors started as "big published renowned authors", they were both amateurs smitten by some earlier works.
And yes, I realize again that in those particular examples, the original work was no longer protected. But that is irrelevant to the question of the derived work's quality.
The Lord of the Rings is sorta-kinda fanfic to Beowulf, the Witcher is fanfic to Slavic and Celtic mythology. Neither of these two authors started as "big published renowned authors", they were both amateurs smitten by some earlier works.
And yes, I realize again that in those particular examples, the original work was no longer protected. But that is irrelevant to the question of the derived work's quality.