HEIF is an abstract concept like 'binary'. Saying that image is stored in binary doesn't tell much, the same for HEIF. JPEG XL can be stored in a HEIF container

HEIF is an abstract concept like 'binary'. Saying that image is stored in binary doesn't tell much, the same for HEIF. JPEG XL can be stored in a HEIF container. It wouldn't help with technical compatibility, and a container format containing everything under the sun is going to bring a lot of long-term support burden and attack surface with it. Also, it is going to bring chaos since no one supports the whole format but just some fraction of it.