virt-manager is big attack surface and gives the users opportunity to mess with settings (either accidentally or on purpose) that could de anonymize whonix.
virt-viewer and remote-viewer would connect to spice already running on whonix and user can only turn on/off simple settings like “clipboard sharing”
remote-viewer source code is also a lot smaller than that of virt-manager
this change wouldn’t affect advanced users negatively as advanced users can just use virsh to edit settings