The reality is that OnlyFans wasn't the first to try this model. You have to give them credit for successfully building the business, especially with several close calls between them and government regulations.
The problem is the payment processor. How the heck do you accept adult-content related payments? That is the hardest problem to solve when it comes to these things in my book.
It's beyond knowing the business model, I guess the founder were at the right place and right time and knew the right people to make this venture succeed.
Also, the marketing, how the heck did these guy blow up so fast. The funds for marketing and all, it's not cheap!