and getting no ad-views and no paid revenue from you is the worst-case-scenario
That is not the worst case scenario. The worst case scenario is I leave Youtube and take with them at least a portion of All my friends (over time) who were not bothering with Ad Blockers anyways.
This means that while me leaving saves them a tiny bit of money on network resources; It actually causes a negative network effect across their whole userbase which reduces their total number of views from people who might have considered paying for Premium or who were not even thinking of the option of Ad blockers.
Your value to a social media site is Not the amount of paid revenue from you.
It's the amount of paid revenue from you plus your network value which includes the value of $$ that can be earned off data learned from you, and the value that can be earned from other people you cause to use the site.
For example, Let's say the average revenue from a youtuber is "X", and the average network value of a youtuber is "P".
If I block ads and never pay Youtube anything, But I upload a video to Youtube that 1000 people come to Youtube to watch who would not have watched on Youtube before, then Youtube has zero paid revenue from me, but then my network value is 1000XP that day; that is potentially 10s of dollars immediately, and tens of thousands of dollars over time.
Ahh, but most people don't upload videos.. Well that is Okay as well, because if I watch Youtube, then I will inevitably end up sharing video links with some of my friends who are Not on Youtube, and referring others to Youtube, then my network value would average (number of users)*(number of referrals)*X*P -- this also adds up to quite a lot over time.
On the other hand If I stop watching Youtube altogether, then my Network value becomes negative, Because It means I will be actively discouraging friends from watching Youtube by referring them to other sources. Me No longer using Youtube will cause my Network Value to the website to become negative, and if it becomes large enough in the negative direction, then it will certainly exceed the hosting costs.
It does NOT cost much to display a video to one person, AND most people will not run effective Ad Blockers anyway, so it can be very detrimental for the site to mess with the few who do.