To be fair, VP9 and WebM both may as well be considered de-facto proprietary formats. Chrome is almost certainly the majority browser and youtube is likely to be the most used video platform. Sending video from youtube to Chrome is probably the main use-case for that protocol. I'm not even aware of any commercial video platforms that offer VP9 as anything more than a novelty option (although maybe Anvato will now that they are slowly being integrated further into the Google universe). Even DASH is years away from real wide-spread adoption.

IMO, video formats will live and die based on hardware support. If your mobile device doesn't come with a AV1 hardware decoder that works with the majority of players out-of-the-box than chances are streaming services won't support it. Apple's advantage with HEVC adoption will be their hardware.