I doubt they’ll ever actually do it, just threaten it constantly.
They’d rather have people without accounts, and/or using adblockers keep using the site. If they started actually cracking down, then it would create a significant pool of users who would use some other platform. They’d rather eat the losses of some people not viewing ads then push a significant amount of users to a potential competitor.
The difference is that Reddit was still largely usable after the API changes and most affected users weren’t getting banned. Some people have a high tolerance for ads, but a lot of people would find YouTube unusable without an ad blocker, and if YouTube was banning people, they’d be destroying a lot of the inertia against going somewhere else.
If we say that something like 20% of users on YouTube are using adblockers, that’s a big enough addressable market for a competitor to seriously take off. Enough to attract content creators and VC funding to get it off the ground. In the end YouTube would probably win, but it would be after a few years of actually having to compete. The revenue loss over that time period isn’t worth the short term gain of getting some small percentage of users to watch ads.