They won't use it to pay their content creators or bloggers. Bitcoin is very volatile, if youtube sent 10$ to the content creator and the market falls suddenly and the value decrease to 8$, most website won't support that way.
I hope that quickly YouTube and other Web sites will start to pay US with bitcoin. I agree with you. I think this will be effective. bitcoin increasingly popular because of the day. but a lot of people hate bitcoin because incorrect information about bitcoin, so this problem is delayed for applied.