You mean a very small percentage of YouTubers say that. Like, not even enough to justify. I'm thinking about it and it seems like the people who would say that are one of three YouTubers. LPers who don't get ad revenue from some games (if you make a LP of many first party Nintendo games you don't get that ad revenue). People who don't get enough views to even justify monitizing their videos (Google only pays if you have more than $100 earned). Or other people who make videos that otherwise can't receive ad revenue from them... Like maybe Team Four Star with DBZ Abridged (as an example). So they say "use AdBlock and just support me via PayPal."
For nearly everyone else this won't work. Big or small. Lets take say Nigahiga for another example. He makes buttloads on YouTube (and pays a ton of taxes on it to boot). Whether or not you like him, he posts videos often that his fans enjoy. If you take away the ad revenue, his lifestyle is going to dramatically change. Yes he has a big following but how many of his fans do you think will willingly give him in donations. You're crazy if you think it'll even come close to what he's making now.
This next point isn't a humblebrag btw, but I'm going to use myself as an example. I have about 5.4k subscribers and 1.5m views across my 90 videos. I'm not too big, I'm not too small. Along with some of my bigger videos I put a link to my PayPal saying that if they want, they can throw any amount at it to support me. The videos that I post this on get at least 50k views. I started posting that link in I think May of 2014. Since then I've gotten 2 people to donate. One donated 20CAD and one donated $5. In both cases it was because I spent lots of time troubleshooting technical issues with them (I make tutorials). I'm not complaining, that's honestly $25 more than I expected and I made sure to reach out to both of them and I became pretty good friends with one. I don't care about the money made via donations. The point I'm trying to make is that if every single user used Ad Block, or Google remove ads altogether, my YouTube income drops down to $25 a year. See how that would affect some people?
And just one last point to prematurely defend myself from the backlash I'm probably going to get. I started uploading videos fairly regularly in May 2011. I didn't start monitizing until September 2013 and I forgot about it. It wasn't until March 2014 that I remembered and got my first payment. I didn't start my channel with the thoughts of making money and I don't want to come across as greedy and entitled. I'm an unemployed and engaged college student using this money entirely to pay off loans early. Would not being monitized stop me from making videos? Hell no. Would it severely cripple my life style? Certainly.