I'll acknowledge my argument is relatively US centered, but when the payment processors causing the issues with adult game dev's having an uncensored platform where they can accept payments and get paid - Visa, Mastercard, Paypal, and the banks where they get their business loans from - are located in the United States, it makes sense to center the conversation on the socio-political issues in the US that are causing the problem.
It would make sense if the real reason weren't international treaties making any money processors and banks accountable if they are aware that the money they processed is linked to an illegal activities. They would be sued and face a millions dollars fine, when not the suspension of their license.
Of course, VISA, Mastercard and PayPal would be able to face the fine, and their license is safe. But it's not the same for banks, that would be the ones facing the lawsuits. And if they aren't big enough, they would lost everything in the process.
That would give them the right to then sue VISA/Mastercard/PayPal for neglicence, since they act as intermediary and should ensure that the transaction is safe. I don't know the terms of the contract between VISA/Mastercard/PayPal and banks, but I'm 99,99% sure that there's no "hey, it's not our responsibility" clauses ; both sides covered their back as much as possible and everyone is responsible for everything.
And here, for VISA/Mastercard/PayPal, it's not anymore millions that are at play, but billions. Not only because of the bank that lost everything, but also because that bank clients would be able to jump on the train ; they lost everything, and someone will have to pay them back.
It's not a bankruptcy due to whatever context, it would be one due to negligence, don't expect a government to come to the rescue when there's an entity that can clearly be held responsible and pay.
Now, what "illegal activity" mean is opened to debate. Are 2D or 3D loli illegal or not, and do they necessarily links with p*d*shit rings and all ? All this is relative. But when what is in play, for banks, is the existence of your company, you ask any intermediaries you've to works with to play it safe.
The same apply directly for VISA, Mastercard and PayPal. If one of them is caught putting a banks they operate with at risk, all the other banks will reconsider their own agreement. And, while they are the three main money processors, they aren't the only one that exist...
Seeing, let's say VISA, collapse because they weren't vigilant enough would be a good incentive. The competitors would immediately reinforce their rules and controls, and of course advertise about it in hope they'll be the one to hit the jackpot.
Then, it's not just content like loli, none consensual sex or bestiality, that would be forbidden. Hell, it's not even just porn that would be compromised, but anything that can possibly look a bit suspicious.