Comment by lxgr

That's backwards compatibility for you.

I think Ultra HDR (and Apple's take on it, ISO 21496-1) make a lot of sense in a scenario where shipping alternate formats/codecs is not viable because renderer capabilities are not known or vary, similarly to how HDR was implemented on Blu-Ray 4K discs with the backwards-compatible Dolby Vision profiles.

It's also possible to do what Apple has done for HEIC on iOS: Store the modern format, convert to the best-known supported format at export/sharing time.