It would also be extremely confusing to support something with only hardware decoding. People running supported hardware would start saying things like "Firefox supports HEVC" which would sound plainly wrong to someone with unsupported hardware.
- You paid the royalties for HEVC, possibly multiple times
- It can almost certainly encode and decode HEVC without breaking a sweat
- You (practically) can't use it ("if it's not on the web, it's dead")