No data for this, but I feel like 9% is less than they expected after 5 years of the "frustrate and seduce" strategy, which is why they're even going after ad-blockers now. If anything, they look frustrated. But they probably had to do this.

Why would they want people using an ad blocker to even use the site that much though. They’re denying them revenue while costing them. I mean it’s great as a user but as a service there’s not really much upside.

YouTube didn't seem to mind so much before. Maybe they wanted to keep those users' attention on YouTube instead of elsewhere. Now they're a lot more focused on the profits, which is fine.