This is extremely important, and something I think many people don't realize. Charging a fee might make it possible for a company to be profitable without needing ad revenue, but it likely won't change the underlying economics such that also showing ads won't be even more profitable. The only ways around this that I can see are 1) a company steadfastly choosing to forgo the ad revenue, 2) a company maintaining a customer base that is willing to pay the subscription fee and has a principled opposition to ads such that introducing ads wouldn't be viable, or 3) somehow reorganizing the economy such that the negative externalities of advertising are greatly reduced.