I think it used to be that Google encoded every video in H.264, but also encoded some videos in VP9: the popular ones. That made a lot of sense--no need to dedicate a lot of encoding power for every video when quick-and-dirty x264 will get the job done fine for >99% of them. And I assume the same will be true for AV1, where only a small percentage of videos will actually be encoded in it. So my envelope calculations aren't going to help me gain much insight.