Does it? Are you sure this is the case for America? Maybe growth internationally, but I never understood how people in poorer countries are able to be monetized as they are in more developed countries.
It does have to slow down at some point. Google now has over 1.7 billion hours consumed on YT a day. That is about 100% growth YoY and has been accelerating. But there is only 7B people on the planet and so there is a limited numbers of hours.
So right now Google gets 10 minutes of every human on the planet with just YT and would expect that to continue to grow but at some point further down the road slow down.
Where do you think it will slow? 30 minutes? Could it ever get to an hour?