Firefox adds tracking protection by default

I don't think it's obvious that's a sustainable business model, although it may be less bad than the alternatives. The Guardian still gets 45% of its revenue from non-digital sources (physical newspaper sales and ads etc), and is barely breaking even after years of losses and cost-cutting, so it couldn't survive on just its digital revenue (subscriptions and voluntary contributions and online ads).

They don't say how much of that digital revenue is supporters vs ads, but they do report "163m unique browsers and 1.35 billion page views" in March 2019, and "over 655,000 monthly paying supporters", so something like 0.5% of readers choose to pay for it. Presumably ad revenue from the other 99.5% is still significant. Now it's a race to increase readership and increase conversion rates, fast enough to make up for the death of print and for the increasing portion of ad money being consumed by ad companies like Google and Facebook instead of going to the content producers. It's still not clear who will win that race.

(Personally I started subscribing to various sites when I considered how much time I spent reading them, vs how much money I spent per hour on books and games and TV and movies and other forms of entertainment. "10€ / monthly" doesn't sound unreasonable in that context.)