If there is a cartel it's at the Visa & Mastercard layer surely.

I doubt Stripe is anywhere near the monopoly status.

In the US, PayPal has nowhere near the adoption of other countries, for making payments directly between accounts, and Visa and Mastercard have enough regulatory capture to ensure it stays that way.

If there wasn't a regulation-ensured duopoly, everyone would be switching to RTP or FedNow which each charge 4.5¢ per transaction, without an additional commission.

No reason it can’t be broken apart in the future. States will likely file suit, as twelve already have regarding the Paramount Warner deal.