Even a co-owner/vice-owner should not be able to singlehandedly demote an owner.
I'm not yet convinced that there's a benefit to having separate classes of owner (original/founder versus other) that outweighs the complexity/confusion that it may cause.
I would say that if an owner doesn't trust someone well enough to make them a co-equal owner then they should make that person a fully-permissioned moderator instead. That of course presumes that the primary change of this topic has been made to prevent such a moderator from making any changes to any owners' role or subscription.
IMHO a moderator should have the power to appoint a successor.
I believe that the ability of a moderator to appoint a successor moderator should be controlled by the permission to Set Moderator Privileges. That would allow the moderator to change a member's role to moderator and set that new moderator's permissions.
