It won't kill the service. The media executives who run YouTube are well aware of how advertising volume affects viewership so they'll titrate up or down as needed to maximize profit.
But don't worry, something else will eventually kill YouTube. Most likely they'll miss some sort of disruptive innovation. Like maybe in 30 years everyone will have content beamed directly into their neutral implants and only a few old people will still watch online videos.
The time line for these sorts of things seems to be: they’ll slowly make YouTube worse and worse, but just not bad enough to kill it. And then something else will come along, and people will be so dissatisfied with the quality of YouTube that people dump it en masse.
But don't worry, something else will eventually kill YouTube. Most likely they'll miss some sort of disruptive innovation. Like maybe in 30 years everyone will have content beamed directly into their neutral implants and only a few old people will still watch online videos.