YouTube makes its money with ads. The more videos a user can watch the more ads YouTube can deliver to the user. Close to 90% (!!!) of all YouTube traffic comes from mobile devices. Therefore it is in YouTubes own interest to allow its users to watch as many videos as possible.
You can be sure that big CDNs will still provide hw-decodeable streams to mobile devices, despite google now offering a performant software decoder on Android. For the same reason twitch not only introduced AV1, but at the same time HEVC as new codec (since HEVC now has broad hw-decode support among mobile devices, unlike AV1).
Funfact: YouTube’s AV1 streams are often considerably larger than the VP9 variant. They are not opting for bandwidth savings currently.
The integration of software decoding is mainly meant as a fallback. As AV1 will become more widespread you will run into cases where there will only be an AV1 encoded version of a video. Not on big platforms like YouTube, but smaller sites. Think the embedded clip in a Wordpress blog or something like that.
Having said that, dav1d is a very very performant software decoder and its impact on battery life is often negligent for the average user.
Land of the free indeed.