> the AV1 video codec was deployed when no consumer had HW decoding capabilities
This was a bug. An improved software decoder was deployed for Android and for buggy reasons the YouTube app used it instead of a hardware accelerated implementation. It was fixed.
Having worked on a similar space (compression formats for app downloads) I can assure you that all factors are accounted for with decisions like this, we were profiling device thermals for different compression formats. Setting aside bugs, the teams behind things like this are taking wide-reaching views of the ecosystem when making these decisions, and at scale, client concerns almost always outweigh server concerns.
YouTube had the same issue with VP9 on laptops, where you had to use an extension to force H264, to avoid quickly draining the battery.
If only they would give us JXL on Android