Sharing answers to top q's we’re seeing about the thumbnail experiment: – Custom thumbs aren't going away – 99.7% still see the creator's custom thumbnail – A lot of creators use auto-generated thumbnails, this test helps improve them for those using them: https://goo.gl/A7oDmr
You do have a strong track record of going from "We're just trying something out" to "It's something you can opt-in to" to "It's something you can opt-out of" to "It's a mandatory feature now".