I'm kind of surprised that they waited this long to mess with ad blockers
If YouTube outright prevents access from browsers that block ads, then suddenly a huge number of the most tech-savvy viewers will be motivated to seek/develop/fund better ad blockers and/or alternative ways to access videos. It's a difficult position. On the one hand, these users are chewing up compute and bandwidth, without providing any ad revenue. On the other hand, if they leave, it contributes to a vicious cycle…
If content creator can not monetize those eyeballs very well there is little incentive to target that niche. YouTube would be losing a niche of a niche that are not as profitable to try and platform.