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Those OnlyFans content leaks, explained
The ongoing issue, and one that affects any site where you can download (and possibly those that only stream) is that some people do scraping.
Not just OnlyFans - ANY site that allows users to download content, esp those that are, "subscribe, and get EVERYTHING!!!", are vulnerable.*
MotherBoard article - very good, very detailed
There are things that can be done to fight it, but it's an arms race, and the good solutions cost money, and the platforms live in a world where content makers chase the highest percentage.
Nutshell - the platform that brings out the big guns, (random frame random location watermarking with specific subscriber info), would probably need to reduce payments to content makers to reflect the costs incurred each time the video is downloaded. POSSIBLY acceptable for PPV, but not sustainable for Monthly Subscription models.
*Content Of The Day. One way that can make it harder for scrapers is to cycle publication so that each day some content can be accessed, and the rest, not. Combined with a daily email, "TODAY, you can see....." it would mean that scrapers have to log in each day, instead of just once. Whether or not honest subscribers would wear this, is another matter.