I think you can leave the decision using SDR2HDR or HDR2SDR to the user. The point is: the two options were present in CE and both worked. Since a few nightlies they do not work any longer and I have not seen any notes in the changelog. were they removed by mistake?
my use case on my Odroid-N2 is:
my LG TV from 2015 does support “Ultra HD Deep Color” st the HDMI inputs (some kind of HDR) , but this is not announced in the EDID-info. So CE sees my TV as a SDR-display. For feeding a HDR movie to my TV I have to disable HDR2SDR. But now HDR2SDR is set to auto, so it is impossible that my TV sees the HDR.
In addition the HDR2SDR function gives sometimes nasty artifacts in the picture (depends on the picture content), which are NOT present in the original movie. I reported this already here, but @anon88919003 said that it is not possible to correct it (see Artifacts with HDR2SDR function)
So please restore HDR2SDR and SDR2HDR functionality as before!