I'm not contesting you, I agree, these don't seem like compelling financial transactions. There's just no reason to assume that such a system has to be a mine-and-then-pay-with-what-you-mined-in-USD-equivalent even though that's all that exists today.

More useful would be a 'pay by letting us consume some fixed amount of electricity by doing some mining with your browser'. The difference between what you paid for electricity and what the company is providing you in products/services determines your cash cost, and is different for every person.

That's all POW cryptos are anyway, buying electricity and turning it into a coin that then sells for cash. The worst case would be if you had to pay in cash because its so slow, so then it became just paying with the coin directly, so why not go back further and pay with the electricity directly and let the publisher deal with mining and coins and such?