Just so that we're talking on equal terms here, card processors and networks solve a massive amount of problems that existed with cash. Pre-card, many stores setup credit accounts with individuals, had book keeping practices to deal with, and had to chase down credit lines they themselves offered. Even with debit cards, card networks facilitate the ability to automatically move money between parties and give customers the ability to dispute/chargeback fraudulent transactions easily. If you physically hand a merchant cash, you can't claw that back without a legal process, whereas you're afforded protections by the card network.

But the 30% cut is completely different from card processors. Of the 30% to Google Play or to Apple, a small fraction (2.9%ish) is actually the card overhead. The rest is split between pure profit, infrastructure, and whatever else gets tacked on.

Cards definitely offer important things to facilitate transactions that are objectively better than a pure cash world for most people. But it's important to call out crazy cash grabs like 30%, which is unheard of even in the payment network world.