I think it's like the fight club formula, if the projected consequence of taking them down is less than the combined sum of all the users missing out on the ads

I think it's like the fight club formula, if the projected consequence of taking them down is less than the combined sum of all the users missing out on the ads, they leave them alone. If their popularity grows, they will take them down. By the way, just because they are available doesn't mean they actually work. YouTube is very effective at working around ad blockers.