Custom thumbnails are not going away! We’re running this experiment to test and improve auto generated thumbnails (a top request we get from creators who do use them). Here's the announcement with details: https://goo.gl/A7oDmr
I’m sure less than 0.3% of channels actually use auto thumbs! The only reason anyone ever used them is because they were mandatory early on. The second you’re given the opportunity, you make custom thumbnails or your channel dies. Auto thumbnails should be removed altogether!