Google wants you to watch ads OR pay for a subscription, but it doesn't necessarily care which; they make money off you either way.
The reason Youtube offers a premium tier at all is to cater to the minority market of time-poor money-rich users who would rather pay than watch ads, which is just a smart move to broaden their audience and diversify their revenue streams. But it's not the primary way Youtube makes money and likely never will be.
Using a $20 CPM [1] (Cost Per Mille, the money advertisers pay per 1000 views), $12 turns out to be 12/20 * 1000 / 30 = 20 ads per day. I would argue that the average youtube premium user watches less than that.
And I would argue that youtube really knows the numbers, and google would not lose money. Don't forget they've turned evil ;)
[1] source is the most recent Big Time video
Actually I suspect the logical operator here is `AND`. In fact, this is largely what holds me back from paying for any Youtube subscription; frankly I don’t trust them to show me zero ads ever regardless of what fee I pay. So I will keep playing the cat-and-mouse game as long as it lasts.
If they ever start doing that, you could stop paying. But not paying now for the hypothetical possibility that they may start serving you ads in the future sounds more like an excuse.
