As currently implemented, I can't see adblocking on iOS taking off very much. It requires 1) knowing that it exists, 2) searching for and selecting an app, 3) installing the app, 4) entering settings.
That's about 3 steps too many for the vast majority of people.
Well it'll be one of those things that every single tech website will run 100s of articles about, become a standard in the list of 'things to do when you get your new iPhone', 'apps to install on your new iPhone', 'things you didn't know your iPhone could do', something colleagues will ask each other over lunch 'hey did you try the new adblocker on your iPhone? It's a world of difference' etc. I think it'll get pretty big quickly.
But I agree, there's no real network effect going on, none of the major browsers will integrate it.
I think the early apps that allowed you to e.g. turn on your camera's light as a flashlight, early calculators etc are probably indicative of how popular these adblockers may get, but those were still pretty damn ubiquitous, not Whatsapp level growth but if you add up all the different apps that did this... it's up there. And we shouldn't underestimate the power of a better internet experience, I still remember when I first got Firefox and then later Chrome, looking back IE was just insanely bad and those two browsers ate up a ton of market share really quickly without any real network effects (although Chrome obviously had a lot of power behind it and even Firefox was marketed pretty aggressively).
At some point in time, Google will face a serious problem, because Google is an ad company. Every service they provide is aimed at serving ads.