I very distinctly remember a dev talk at early 2000s Microsoft where a distinguished engineer recently come to MSFT from VISA described the herculean effort that it took to run this network. There was an anecdote that he shared that stuck with me where they had a worldwide daily balance mismatch of something like 0.37 cents and it was all hands on deck to find the missing thing. Yeah, man, it is very difficult to run these networks, hence so much money in it.