They need people who won’t watch ads to stop using it to lower their costs.
This assumes that the biggest cost to Youtube is serving the content, not storing the content. I'm not saying you're wrong, but I think it's a valid question because if storage is the larger cost, then it doesn't matter how many visitors visit the site, Youtube is still warehousing all that content. By the way, in that scenario, it's actually better for Youtube to keep as many viewers on the site as possible, adblockers or not, because they can use higher viewer numbers to increase the price of the ad space they charge to advertisers.