I'm a software engineer at AWS and work on video content delivery for services like Netflix. The idea that one single ad could cover the cost of delivering a video that's been replicated in multiple servers, multiple regions, multiple countries throughout the world is pretty hilarious. No matter how much money you think YouTube is making I can almost guarantee it's not enough. There is a reason there is no significant competition in this space, it makes no money.

If you think it's sustainable you can create a new service yourself, no one is stopping you. I've done cost estimations for projects with 1M+ customers and the margins are so tight we've killed at least a dozen services despite pouring months or years of effort into their designs and prototypes. It's easy for you to complain about freebies from your couch but the reality is that if someone could make a better service than YouTube, they already would have. "Spare is your tears" lol.