We don't know that. In fact, the product has existed for almost 10 years, and they still haven't added ads.
The obvious reason they haven't added in ads is that the subscription model is more profitable, and the lack of ads is the only reason anyone is actually subscribing. The moment that stops being the case and they start showing ads on videos again, even at a lower frequency, is the moment they burn a $10 billion / year business to the ground.
So I don't think it'll happen. But if it does, so what? I'll just cancel my subscription for the future, and likely stop using YouTube entirely. It won't erase the benefits I had from the subscription until then. Like, they can't make me retroactively watch ads.
The obvious reason they haven't added in ads is that the subscription model is more profitable, and the lack of ads is the only reason anyone is actually subscribing. The moment that stops being the case and they start showing ads on videos again, even at a lower frequency, is the moment they burn a $10 billion / year business to the ground.
So I don't think it'll happen. But if it does, so what? I'll just cancel my subscription for the future, and likely stop using YouTube entirely. It won't erase the benefits I had from the subscription until then. Like, they can't make me retroactively watch ads.