Maybe we could do like this in base-files.bb ?

FSTAB_HAVE_WIC = "${@bb.utils.contains('IMAGE_FSTYPES', 'wic', 'true', 
'false', d)}"
FSTAB_WIC_UPDATER = "${@bb.utils.contains('WIC_CREATE_EXTRA_ARGS', 
'--no-fstab-update', 'false', 'true', d)}"

do_install:append() {
	if [ ${FSTAB_HAVE_WIC} = true ] && [ ${FSTAB_WIC_UPDATER} = true ]; 
then
		sed -i '/\/dev\/root/d' ${D}${sysconfdir}/fstab
	fi
}

This would remove the hardcoded '/dev/root' entry  if all conditions 
are met to add the root entry with wic fstab-update
> 
It˙s looking like the easiest fix here is to revert your patch, so 
> at least we don˙t have warnings and unexpected behaviour on boot.  
> Unless you˙re willing to rework your patch so that it allows edits 
> to the root mount point but doesn˙t end up creating duplicates?