That's a high standard for being considered "ethical". I'd argue that as long as the platform incentivizes voluntary work more than forced work, it goes in the right direction and is thus "ethical", or at least not "unethical".
Note that I am not claiming Onlyfans is unethical. I don't know if Onlyfans is ethical or not. I don't know what the rate of abuse content on Onlyfans is. That's information I simply don't have. Furthermore, I'm not even sure what rate of abuse content society should tolerate from a company. Probably if these are only isolated incidents that are swiftly recognized and rectified, it shouldn't damn the whole platform. Also, I think it probably matters whether that rate is trending up or down, but I don't have any information about either possible trend. I lack the information I need to support the claim that Onlyfans is ethical, and I think the rest of you do as well.