AV1 (and AV2) do save a lot of bandwidth, and Youtube already serves AV1 selectively to some users. But the biggest drawback of AV1 is that it needs more resources to even decode. This is especially apparent on old computers that don't have dedicated AV1 encoders or decoders. I discovered this when someone complained to me that PaleMoon on their old PC suddenly consumed a lot of CPU resource causing the PC to stutter - turned out that Youtube was serving AV1 encoded videos on it. Disabling AV1 support in the browser fixed it as Youtube was then forced to serve AVC / H.264 encoded videos.