They could, but things like sponsor block already exist, and if they disallow slipping ads, you can still have an extension that blacks them out and mutes them

They could, but things like sponsor block already exist, and if they disallow slipping ads, you can still have an extension that blacks them out and mutes them until they are over. But even then it goes indefinitely deep: How about a video player that skips ahead once the buffer has enough frames and then plays the ad in the background while showing the caches video? So you need to disable seeking entirely. But what about an extension that preloads videos in your timeline to a buffer of the average ad length and then play from cache and put the ad in the background. So you have to introduce DRM, but I can do the thing above from outside the browser... I suppose you'd either need to lock down the entire system, have crasy detection algorithms or make the bar just high enough that most people won't bother.