Which users actually need and want irreversible transactions. What's the utility of a system that doesn't let you combat fraud?

A system with a middleman capable of reversing transactions enables fraud. See also: the folks behind Flipper Zero being permanently robbed of more than a million USD thanks to PayPal's whims (as the latest of a long list of high-profile cases of PayPal taking vendors' money and running).

people targeted by law enforcement or secret police need unstoppable transactions