This depends on the quality you are aiming for. You just cannot reach the best quality with 8 bits, not in SDR, not in HDR, not with gainmap HDR. Sometimes you

This depends on the quality you are aiming for. You just cannot reach the best quality with 8 bits, not in SDR, not in HDR, not with gainmap HDR. Sometimes you don't care for a particular use case, and then 8 bits becomes acceptable. Many use cases remain where degradation by a compression system is unacceptable or creates too many complications.