The Hivemind ruler is immortal by virtue of being the result of all the non sentient pops/the sum process running on all that meat hardware.
The only way to 'kill' a Hivemind Ruler would be to kill, conquer or free all it's drones.
Kill 99% of the servers of a cloud application. You might eliminate 99% of it's storage* and 99% of it's processing power, but it will not stop that application from running until you killed 100%. Continuity of existence as long as one drone remains.
No fallen empie can "Assasinate" a Hivemind Ruler upon winning a war.
Age will not kill it, only make it repalce a lot of the nodes.
AI rebellion sabotage can not kill the leader. It is not a single entity to kill.
If you kill 99% of it's pops you killed 99% of it's production and research capacity**. But you did not kill "it".
*Storage rarely matters for cloud/distributed applications. Unless it is a storage cloud. At wich point there are propably less volatile backups.
**The technology base can be easily held in even 1 pop unit.
I am aware of how that works...
What I am saying is leaders are themselves capable of transfer their conscience from old body to new body just like their ruler are.
Just because they are autonomous doesn't mean they can't do it as well. But they, autonomous drones, have fewer body that their decision making process is being run on.
To use my favorite example of a Hive Mind, MorningLightMountain, albeit bit more cybernetic orientated than current Stellaris in-game mechanics allows.
Now image a Hive Mind civilization in the stone age and no innate pseudo PSI with two distinct members of the same species. One is basically the foot farmer/soldier/other brawn roles and to make it simple I will call this one motile. The other one is the brain also known as immotile. Now the brain is actually "four motile" merged into a single body discarding their arm and leg. The immotile pass along "to do list for the day" to each motile it control. So motiles doesn't really have any meaningful autonomous at any point whatsoever.
Now fast forward to space age. The immotile long ago stop using physical contact to pass along commands and instead turn to cyber implants to allow long-distance communicate with their motile. Image the Hive Mind with a matrix like jack directly to their nerve that receive pulses. Then turn it to radio wave or something much easier to transmit over long distance. Then the motile act on that command as soon it get it.
Now to work around the problem of radio wave taking years to reach, assuming no FTL communication which Stellaris has, their long distance immotile cluster or motile etc... They basically made the various immotile cluster capable of making it own decision over short-term strategy/tactic and other not so important things.
So long story short. A Hive Mind is capable of having autonomous members, leaders to use Stellaris name, that are also immortal themselves or to put it another way there is more than one copy if smaller than original Ruler running in parallel across the entire species with different scope of responsible.
IE Admiral only pay attention to his own fleet operation while governor deals with logistic/nuance of running a planet. With a personality to over-watch all of it at once.
