Sponsorblock would be easy to work around if ad-breaks were randomly selected. Whenever you upload a video YT could process the video and "search" for good ad-break locations, at display time client bits get pushed ads randomly, making time-code blocking useless.

They could even upsell to content creators by giving them the means to dynamically replace sponsors in old videos using the same tech, maybe with some knobs and switches for content creators to select where sponsor breaks occur.

Maybe someone could use train a AI for something useful like skipping advertisement? Switching from time to content id, i.e. like they use for copyright strikes, would work even better than status quo.

Sponsorblock tagged videos would serve as excellent training data for an ad-detecting AI.

I guess something could detect if the frame shown is a frame from the video and if not skip

The ads don't always have to be injected into the exact same parts of a video, and YT can also choose not to stream any more bits until it thinks something's been "watched".

Sponsorblock only works because YT allows you to skip anywhere in a video.

This. It's why in-stream ads were so straightforward to implement for Twitch too. Can't fast forward a live stream.

If you're willing to buffer a few minutes of video, you can work around this. Tivo is a decent example of how this has been done in the past.

Oh yeah, that works. Cable TV + Tivo is a good analogy for all of this.

Even if they try to prevent that, with a modified client, you can skip anywhere in a video and there's nothing they can do about it.

That requires them to pre-serve you the bits for the part of the video after the ad, which they aren't going to do.

Unless they want to completely break the ability to seek in the video timeline, regardless of ad placements, that's not going to work.

It already behaves this way if you aren't blocking ads. If you seek too far ahead in a video with mid-video ads, it'll show an ad before playing at the timestamp you selected. If they want to prevent ad-blocking, they can do the same thing except not serve the video ahead of time.