There's no incentive for them to do this though. Cable TV uses a broadcast model, so it's possible that ads on subscription channels is a profit-maximizing strategy. But YouTube users have independent streams of video. If some users want to pay nothing and see ads, while others will pay a little for fewer ads, and still others will pay full freight for no ads, then that's a profit-maximizing strategy and it will happen.
There's precedent with Youtube Music Key. $10/month is a lot to pay for removing ads from music videos alone, but given that it's bundled with the overall streaming service, it's a pretty good deal.