But Twitch is already established as a billion-dollar company (acquired, no less) providing SFW streaming services. I could understand staying out of the porn market in their case. But OnlyFans is _only_ associated with erotic content.

It's corporate suicide and it makes zero sense that they aren't fighting it. It's a valuable market, it's ethically sound and they are making the world a better place by having a safe place for adult performers to sell their services.