I can't disagree that they have a moat, but it's a hard problem and if it were as easy as you say somebody would be disrupting them already to get a share of that $24T.
Just dealing with fraud is a major problem in itself.
The problem arises when the banks are in two different countries. If money leaves your account and mistakenly ends up in an account in another country, even within Europe, it can be very difficult to recover.
That is why national payment systems tend to work relatively well. Cross border systems are a completely different challenge.
Just dealing with fraud is a major problem in itself.