Is that such a bad thing? Are they really providing that much value?
The remaining YouTube channels would be concentrated around the ones that are of higher quality, rather than easy slop produced to push ads. Nobody would try to clone someone else's channel for money. They would only be produced by people who were passionate about that topic. There would be fewer channels by passionate people, but the percentage would be much higher, so it's not necessarily a worse situation overall.
TV has always cost money - you pay for satellite or cable, and the free-to-air programming available to you is overtly subsidized by your government - they shouldn't need to double-dip by showing ads.
We used to pay for newspaper subscriptions too. A lot of newspapers are trying to go back to that, but it's a market for lemons. Maybe if advertising was banned, we'd each subscribe to one or two online newspapers and discover which ones are decent and which ones are crap. Remember, it's harder to make someone who was paying $0.00 pay $0.01, than to make someone who was paying $10 pay $20. Would the market be more efficient at price discovery if there was no $0.00 at all?