What about international travelers who don't have an internationally accepted payment card because their home country was mostly isolated from the banking system in a mostly futile attempt to punish its government?

I don't think cash is saving anyone from the knock-on effects of international sanctions on their citizens. The same people who don't have access to a Visa because they're citizens of a sanctioned country aren't in a position to easily turn their local currency into USD, and the ways they'd start to earn money outside of the sanction bubble overlap the ways they'd get an internationally accepted payment card.

You can freely exchange Russian rubles to US dollars and back. Many people I know travel like that.