The core intention of YouTube is to have a platform to advertise on. Without the viewers, the advertisers would have no interest in the platform. If there were no advertisers, that would not sway the viewers from having interest in the platform. The ads are there as a means to the end, it’s not the end itself.

If there were no advertisers, they would not be able to pay the expenses required to deliver something that meets user's expectations, and they would lose interest.

Not necessarily. There are other models that could work for YouTube, like the subscription model, and they could always go back to YouTube being an unprofitable arm of Alphabet like it was.