@brooke oh hang on, we can spreadsheet this trade-off.

Let's assume that your other option for UHD playback is AV1 but on a device other than the AppleTV - say, Kodi on a MeLe Quieter4C. Those cost about, what, $200?

Whereas that same money put towards more mechanical storage nets you... quite a few more playback hours at HEVC compression rates. Plus the convenience of not juggling multiple devices.