I don't think it follows that they are willing to pay some amount of money. Maybe they're willing to pay for something like Google, Wikipedia, Youtube as a plat

I don't think it follows that they are willing to pay some amount of money. Maybe they're willing to pay for something like Google, Wikipedia, Youtube as a platform, but they're probably not willing to pay for most of the niche and small sites. And I agree that pay-as-you-go would put a high mental overhead on sites. That's why we have ads instead.