The system is designed so that if there is only one system administrator, that user cannot be removed, or have those system administrator abilities removed. And a good thing too!
But normally, most businesses keep two system administrators on the system. One is typically the real administrator, doing the bulk of the work day to day. The other is typically a senior manager who keeps an eye on the system but does not typically enter much data. This is also the person who makes the administration changes to the system if the everyday system administrator is terminated or resigns and has to have their system access disabled.