I don't think that someone is talking about using BTC or ETH. They're literally unusable to make purchases (multiple minute settling times and the high costs you mentioned) but any other functional L1 would work. Algorand for examply has ~3second block times with instant finality and costs a fraction of a fraction of a cent to use the network. Those fees couple be subsidised they're so low. They're significantly lower than a rail like Visa or MasterCard. But you're right, implementing it so anyone can easily use and access it is key.

Edited bad autocorrect