Well, until you realize that the merchants affected in this specific incidence, itch.io and steam, have had 12 and 21 years respectively of operation without causing issue for online payment systems, indicating that this is a BS excuse.

If the payment systems find that 90% of the fraud/chargebacks through these platforms is from 0.1% of the games, of course they would push for those 0.1% to be banned.

Why should that be the case?

Do the credit card companies go to Wal-Mart, find all the fraud is people buying cigarettes, spirits and razor blades, and tell Wal-Mart to stop stocking those?

Of course they don't.

The payment networks don’t pay for the cost of chargebacks - that’s borne by the merchant or the card issuer, the network makes money either way.

This is one of those situations where the optimal level of fraud isn’t zero.

Is there any evidence that this is the case, though?

Who does chargebacks on Steam? They'd lose their whole account.