> Maybe people just got fed up with Youtube.

This specific case is about an unusual high drop of viewers specifically on desktops on a specific date. The assumptions are, that it's just too unusual for the normal drop in that timeframe, so it has to be a bug of some kind. Would it be a normal drop in viewers, it would not be on a specific date, months after the problems with AdBlocks started.

There is middle-ground: anti- ad-blocker changes cause a large number of ad-blockers to fail entirely.

It would make sense too, Youtube wouldn't care to make their videos viewable to a large number of ad-blockers, and ad-revenue would be near steady because ad-blockers were not generating any ad revenue.