Ann: Mogul Reaches Milestone of 1 Million Registered Users, page-23

Whilst I see where you are coming from and agree with your point that not all users will pay, the article does state that Youtube Red is not available to the full 1B:

YouTube Red has only rolled out in four countries so far — US, Mexico, Australia, and New Zealand — which means the service isn’t available to all of YouTube’s over 1 billion monthly users. 

The article also makes another good point, Youtube has been around for a decade for free, try convincing anyone to start paying $14.99 for a service they have had for free that long. Especially when the service is similar to products consumers are already likely using (Stan, Netflix etc.).  

Mogul has rightly started without cost to drive user acquisition, however, if the product is good I am sure most users would not have an issue paying a small fee.