As long as every institution that's required to make the payment happen buys in (and it is compliant with whatever regulations are involved for sender, recipien

As long as every institution that's required to make the payment happen buys in (and it is compliant with whatever regulations are involved for sender, recipient and intermediate institutions) one does not have to use SWIFT explicitly. That said, gruturo above (in our discussion about the associated ISO standards) makes a good case for why most banks default to SWIFT.