I'm not sure why they would fail if they really wanted to.

From what I understand, currently, the ads are distinct videos, which is why external tools can block them. For example, you don't get ads with youtube-dl.

But if Youtube used the uploaded video to create a new one, with ads injected in it, I can't see how adblockers could block that reliably. Perhaps you could build a database of timestamp of ads per video? And hope that the compute power required to build new videos with ads in it is too much to be done widely?

What am I missing?