Relevant statement from main Incus dev:
stgraber4:34 pm
The root disk must always exist and is the only one that gets snapshotted, backed up and moved around as part of the instance.
For VMs, you can make it tiny and never use it, instead relying on external disks (custom block volumes) for everything, but you can’t fully remove the root disk as it’s assumed by a lot of different code paths to be present.