There's no need for a worldwide solution, federation and interoperability between major schemes are enough.

The same way AliPay can be used in most Asian countries already, we can imagine a world where EPI, UPI, Pix and so on interoperate smoothly.

I assume Visa and MC will try to remain walled off as the US are a big and slow mover in this space, until they'll need to open up as well.

Note: payments (unlike transfers) are never really zero fees, for good reasons.