EDIT: I do wonder if the price point of youtube premium is too high to capture an audience that simply doesn't want ads but doesn't care about any other features. $6/mo feels higher than the typical per-user ad revenue.
EDIT2: While I don't know youtube's average revenue per user, both Facebook and Google's ARPU for the NA segment is ~$100-$200/y, which would put $6/mo quite close to the replacement cost of lost ads revenue.