DisplayPort 1.4 only supports 25.92 Gbps of data (50% more than DisplayPort 1.2) but has Display Stream Compression (DSC) which is required for 8K 60Hz 8bpc (RGB or 4:4:4).
HDMI 2.1 supports 42.667 Gbps and can achieve 8K 50Hz 8bpc (RGB or 4:4:4) without DSC (60Hz requires DSC). HDMI 2.1 also supports DSC though, so it can go much higher than DisplayPort 1.4.
HDR uses 10bpc.
So even though the bandwidth of HDMI 2.1 is greater than DisplayPort 1.4 (only by 65%) an adapter to convert one to the other (including DSC on both ends) would still be useful.
Is it possible to create an HDM 2.1 display that doesn’t support DSC? If so, then it would be useful for a DisplayPort 1.4 to HDMI 2.1 adapter to tell the computer that it supports DSC, then uncompress the output to the HDMI display.