The main problem with this analysis is that not all Youtube viewers are of equal value to advertisers. Premium subscribers are the people who have demonstrated that they are willing and able to spend money on luxuries. These are also the primary audience of advertisers (compared with, say, the elderly living off welfare, minors without a credit card, people living in poor countries).

Every premium subscription Youtube accepts reduces the value of its ad-supported audience, not just in an absolute sense (i.e. this user won't watch ads anymore), but in the sense that it lowers the CPM advertisers are willing to pay for the remaining “cheapskates”. The premium subscription price has to account for that, which is why the price should be significantly higher than the average ad revenue per user.