Current cryptos are too slow to process and have too many onboarding steps many of which are also quite slow and require more involved from the customer which increases drop outs.
You can’t buy crypto with a credit card easily, even on the big exchanges and you have to go through a KYC process and then use debit or bank transfer usually.
Also Onlyfans is a British startup I’m not even sure it’s hosted in the US.
Onlyfans is really not a private cryptos anything. It’s $6b per year, the yearly purchase rate of Mastercards network is about $1.5 trillion.
It is not going put MasterCard out of business just like PornHub didn’t.
Also at least in the UK I can open a bank account with a utility bill to get a crypto account on any major exchange I need to upload a photo ID usually a passport as well as a proof of address.
Then I need to make a bank transfer and even then the account is still usually limited till some arbitrary time frame has passed or number of transactions have occurred.
Transferring crypto is still much slower than using a card. Get the address / scan the QR code, log into your exchange, do the transfer and then wait until X number of confirmation has passed which can take 20-30 or even more minutes.
This is antithetical to an impulse buy which quite likely how most of the people that pay for Onlyfans and other porn sites get hooked in the first place.