If the price of advertising goes up, Google will be incentivised to water down Contributor. An equilibrium might form, but in the long run it probably won't be "ad free". Google can't get away from the contradiction.
> I'm not sure that's a "walled garden" in any meaningful sense.
Hence "semi-walled". It's walled in the sense that only Google's ads are affected. They can somewhat reliably track users on sites that belong to Google's network. They can't do so otherwise. I imagine they solve fraud with their existing tools.
> If the price of advertising goes up, Google will be incentivised to water down Contributor.
The Contributor model, as I understand it, intrinsically scales costs (or effects) to the cost of advertising, with participants effectively bidding against advertisers for their own views. So, no, they wouldn't be incentivized to water it down if the price of advertising goes up, rather, the fact of Contributors success would drive the market-clearing price of advertising up while driving up Google's revenues from its ad networks, simply by increasing the market for its ad networks from "advertisers" to "advertisers + contributors".
> An equilibrium might form, but in the long run it probably won't be "ad free".
It will probably be ad free for the users willing to pay the most money to avoid ads, and less so for those willing to pay less.
Which, ultimately, is a pretty reasonable market-based solution. You choose the content you wish to consume, and you choose the degree to which you'd prefer to pay money vs. viewing ads.
> I'm not sure that's a "walled garden" in any meaningful sense.
Hence "semi-walled". It's walled in the sense that only Google's ads are affected. They can somewhat reliably track users on sites that belong to Google's network. They can't do so otherwise. I imagine they solve fraud with their existing tools.