There should be an option that a fleet admiral can set someone the next in command to be fleet admiral so that a fleet will always have someone who control of a fleet if the creator/Fleet Admiral disappears for what ever reason. I was one of those who were in a fleet when the Fleet Admiral and left the fleet. It was why I even created my fleet in the first place.
While I agree with what you are saying I would say this, a good fleet will already name it's next in command and give them the appropriate rights. I can tell you from experience the fleet I am in had a fleet admiral leave and our named next in command was able to take over and name a new next in command. When that fleet admiral then left the process repeated. Despite now having two fleet admirals leave we still have two people on top of the chain of command and a succession plan should it happen again. Really just comes down to having players in the fleet you can trust with those types of responsibility.