Yes, surplus is a thing, I agree. But that doesn't materially change what I said. The thing still has to be worth at least as much as it costs for users to be willing to pay for it, so what users will pay at least sets a lower bound on what the thing is worth to users. (Note that it can be worth different amounts to different users; the more precise way of stating it would be that in a competitive market, price equals marginal cost equals marginal value, i.e., value to the marginal user, the user who just breaks even paying that price for it.)

> What ad-supported services did is zero out the price of anything that costs less to provide than the amount of ad revenue it generates.

Which also uncouples the price from any measure of value to the user. The price is now measuring the marginal value to the ad purchasers. The value to the users can be anything greater than zero--the fact that they're using it at all means (or should mean, if the users are rational) that the value to them is positive. But it could still be less than the cost to produce. And the worse the user experience gets, the more likely it is that the value to users is less than the cost to produce, even if that cost is small.

Plus, there's a whole other piece of this that the analysis we've just done doesn't even capture: externalities. One simple way of stating what many people think is wrong with the ad-supported business model is that it creates large negative externalities that, on net, mean that the value to users is negative--but the users don't see the externalities so they don't realize this, and the tech companies have offloaded the costs of the externalities onto others, so they don't see them either.