This makes a lot of assumptions in the same way that "pirating loses game companies money" - yes but its not always clear cut or an exact science. Pirating was also a big factor of Minecraft's initial popularity.

What if adblockers contribute to a video going viral? Or even sharing a video with family/friends who may not have an adblocker installed? What if I'm a content creator who uses adblock and now I share my content elsewhere? What if I'm a content creator primarily funded by Patreon and many of my viewers were adblock users and I move to another platform to retain my patrons and now YouTube loses money on all my subscribers who weren't adblockers? What if enough people are exiled that the go to place to watch new videos is no longer YouTube but Vimeo or another competitor? Social media sites rely pretty heavily on network effects and if enough users leave (Myspace, Digg) a competitor will fill the space (Facebook, Reddit).

The gamble here is that more people will subscribe than people who will leave. Which is likely true but it could have knock on effects.