Is there an reason why only the owner of the dashboard can edit the dashboard?
From a security standpoint it adds an additional 3 steps per dashboard to edit in the event that an user accidentally share a dashboard with the Global ("Everyone") setting. An admin cannot just edit the dashboard to change it to a compliant share setting, without changing the owner, then find the dashboard, edit the permission, then change the owner back to the original owner.
Additionally would it be possible to add a GET, DELETE, POST /rest/api/2/dashboard/{id}/permissions/ endpoint where an admin can systematically perform the permission cleanup?