If it's a requirement to enter the market, why wasn't it considered when developing the business model?

This is absurd. In any other situation you’d never argue this.

Ok, so I can just ignore all regulations because I don’t like them? I can sell unsafe food in my restaurant because I don’t want to agree to the terms the government says I must adhere to?

It's not absurd. Have you never heard the term "contract of adhesion"?

And contracts aren't the same as laws and regulations, so throw that strawman out. Laws and regulations are enacted through a process designed to serve the greater good.

They are not required to accept them, OnlyFans is doing it just fine without Apple. They chose to be in Apple's platform. Voluntarily.

What market? The market of iOS users?

There are literally more users on Android - If I were in that position, I could just switch platforms.

If I'm a radio manufacturer, I can't cry foul that Lamborghini won't allow me to sell my radio in their cars by calling them a monopolist over the "market of Lamborghini drivers."

If I then decide to still make an exclusive Lamborghini radio, I own that responsibility for my failed business model.