The exact correspondence between the VPx-es and the MPEG codecs is not that relevant, because old codecs don't go away when new ones come out. In fact, hardware decoders are irrevocably baked into devices where consumption happens, so anything H.264 or newer becomes difficult to displace -- like Google's ham-fisted push [1] for VP8 and VP9 on YouTube for their own benefit, burning through people's batteries in software instead of serving H.264.

But I'd argue that the MPEG codecs roughly slot in between each tier of VP8, VP9, and AV1, because the MPEG codecs are tremendously flexible (as can be seen in Netflix's tests [2]), have dozens of competing encoders of different qualities, and dozens of profiles that constrain or unlock advanced features. So in a way, it can be simultaneously true that AV1's competition is both H.265 and whatever comes after H.265.

[1] https://news.ycombinator.com/item?id=13230676 [2] https://medium.com/netflix-techblog/more-efficient-mobile-en...