Just not showing content when there isn't enough adds available surely would drive away watchers. And the advertisers are already bidding. Certain types of content get higher pay.
Imagine you are inside a store that has magazines. The magazines are all free, because they are filled with lots of advertisements. Then someone says "It's crazy that these magazines can't set their own prices". And someone else says "How would you do that, nobody would pick up the magazines".
The magazine might be free. But it might set the prices for adds so high that it don't get filled. Or actually make money. And some VC keeps throwing money at it as surely at some point they will corner the market and make money.
And the video makers and article writers can already contact advertisers and ask for whatever price they want.
Hmm, that could work. And also Youtube as platform could charge you for delivered content. So set high per impression price and if you get more views than add revenue Youtube would send you a bill. Could work, could work.
