Replying to self… I'm wondering if the recent per-country daily "Top 10" that Netflix is now exposing in its UI (now scraped by https://flixpatrol.com/) is not a response to ISPs already building this ? Like a way to kill two birds with one stone: reduce value of private measurements, and improve UX at the same time.
Also you don't need a precise measurement. Just measuring a random 10k users in 1M network daily will give you more precise viewership measurement than anything Netflix publishes.