Good point. A rough estimate would be she gets 10% of each of the 600,000,000 $15 Harry Potter books that have been sold worldwide. That comes out to $900,000,000.
A lot of that value came from the many workers involved in the publishing, marketing, translating, printing, shipping, etc. of the books. Did she get the surplus value of their labour? Does intellectual property count as means of production? How is that different from Youtubers and Twitch streamers benefitting from the labour put into their respective platforms?
If intellectual property is valid in at least some sense and she deserves monetary compensation for the sale of Italian translations, why does that not transfer to the licensing for movies and theme parks?
Btw I'm neither defending Rowling nor am I being antagonistic / trying to pull a gotcha or anything. I just think it's interesting.
(This is just my interpretation of the proletariat as a class and intellectual property, so take it with a grain of salt.) It's worth noting that in the post title's case, the labour embodied in YouTube as a platform and the advertisements shown by YouTube would have existed without the video creator in question. In Rowling's case, the vast majority of her wealth comes from her ownership of the IP, which is then "leased" by the corporations that produce the content in question (books, movies, plays, games, theme parks, merchandise, etc.), meaning that the source of her money is effectively rent. Since the IP isn't something tangible like a housing complex, I'm not sure if people like her are clearly bourgeois like landlords, but she's certainly not surviving by selling her labour power (so she's not proletarian) and she's not surviving by selling goods she created (so she's not an artisan)