Without hardware attestation/DRM, all detection methods served by the server can be rendered futile. It's an inherently lost game for YouTube.

Why couldn’t they just introduce DRM?

They already use it for paid “commercial” content on their site (like TV shows and movies, whether pay per view or in a subscription).

There are zillions of youtube client devices out there that rarely get updated or no longer get updated at all. It's tricky to 'introduce' a lot of things, never mind DRM.

YouTube has infinite time and energy for this. Adblock developers do not. Some are already burning out. The biggest risk to YouTube's strategy isn't that they can't win technically, it's that the bitching from their employees internally grinds them down.

Because we own the client.

You still have to be able to tell the difference between content and ad. That seems like it will become hard soon.

SponsorBlock will help.