Short answer
HubSpot's KB: users with Add and edit users permissions can only assign permissions they have, so you must be a Super Admin to assign Super Admin. A Super Admin can do virtually anything; a user with all permissions but not Super Admin still can't do some things. Removing Super Admin leaves the user with whatever permissions you assign.
1. Users and Teams
User settings in my demo portal.
2. Keep two
Having more than one Super Admin avoids being stuck when one leaves.