They've a/b tested inserting ads into the video stream before. If most people acted like I did, then YouTube would have seen them come back once every week or two and leaving as soon as one of those ads came up.

Right now it seems like most content providers are actually experiencing competition as it's extremely easy to jump to the provider with the least onerous ads, while torrents create a floor in terms of how annoying your ads/drm can be before people pirate