Crypto payments aren't cheaper than card payments
If crypto did all the things that the law requires card payments to do like (in the US at least) zero liability for fraud, and the fraud prevention they therefore feel compelled to do to limit that potential liability, police PCI compliance of vendors, and on and on, it would cost a lot more.
If the cost of actual transactions was basically free like card transactions are to Mastercard and Visa then the only difference would be the profit they make. If the profit is so high why have other card networks that take less profit not popped up? It can only be a combination of high barriers to entry (all the compliance, regulation, fraud prevention etc. a new entrant would have to set up) and whatever monopolistic barriers are set up by the current incumbents.
Crypto doesn't magically make all that stuff go away. Sure, they can operate today without any regulation, but long before they become a noticeable drain on Mastercard and Visa's bottom line they would be forced to comply with all the same regulations and the "decentralized" thing cryptobros claim to love so much would be a thing of the past.