That would not work, because of the following (quite neat) problem: The users that are most valuable to advertisers are those with a lot of disposable income.

That would not work, because of the following (quite neat) problem: The users that are most valuable to advertisers are those with a lot of disposable income. If you offer an ad-free experience for the value of the average user, far more of these high-income, high-value users would subscribe, both because they have the money and because they tend to be more privacy-conscious. Imagine (simplified) losing ad income from the top 50% of your users for the price of your average user, while being left with only the bottom 50% to show ads to.