The site hosting the ads seems like a red herring? Do you mean they could sell or design their own ads? If so, that seems like a difficult proposition.

If we accept that most people won’t pay a subscription, and take ads off the table as an option, then I can only think of 2 other options:

* charitable patrons (this is a thing, but I guess not effective enough?) * selling other products to subsidize the free content

These both appear to have obvious problems and for a dubious goal of making another party subsidize the visitor’s consumption cost.

Having the visitor cover their own cost seems reasonable. What currency do they have other than money or attention? Maybe a small work problem that provides an abstracted service to a separate payer (a la reCAPTCHA, but for $).