I've extended QOI for 10-bit here: https://github.com/AuburnSounds/gamut (and also grey and grey+alpha). So it's a separate incompatible codec yes.

The problem with a QOI that would have 16-bit is that lossless becomes more expensive, exact match in the color table is more rare too and not worth it anymore.

You will start to need more prediction modes + offsets.

Using QOIX for elevation maps in PBR currently.