I was partially responsible for designing a system that could be used to eventually scale to handle processing every global credit and debit transaction.

GPS is harder.

disclaimer: I used to work at Visa.

Were you also partially responsible for designing a GNSS system? Otherwise it isn't clear how you can make any claim as to the difficulty of that, if you're using your experience on the former as your bona fides. I think the difficulty of payment processing isn't in the happy path, but in fraud/anomaly detection, merchant servicing/outreach, dispute resolution, all of which do not have a "set it and forget it" solution.

The platform had to encompass all of those payments infrastructure responsibilities. I'm quite aware of their relative difficulty.

Separately, I did not have to implement a GNSS system, but at the job I had immediately after Visa, I did have to work very, very closely with GNSS vendors who integrated with our AV middleware software (including having to write drivers for them), and in my current role I'm CEO of a company which produces a software platform that provides automated V&V of complex SISoS (including those integrated with and dependent on GNSS).

End to end payments systems are hard in the rats nest of legacy expectations and integration sense. End to end GPS deployment & management seems hard in the science and systems engineering sense.