If they were gods, immensely powerful in themselves, or had a legion or impervious warbots to back them up. Otherwise they'd need the agreement of their supporters. The social contract, however it develops in a state, is the thing that supports the leader.
Look at what happened in East Germany at the end of the Cold War.
Erik Honecker was an immensely powerful leader, but when he lost the support of the political structure he had climbed and led for decades, he fell rapidly.
If the Emperess can live forever, what about the Moot members who are required to confirm the power of the throne? What happens to the offspring of the Emperess? How is an heir designated after that system takes place? Would they wait through various bodies until some cataclysm clears the way for their rise? What about the security system commanders who keep order for the leaders - do they get a bite of the cherry too? What about their subordinates: will they all just accept a shortened life because the Queen wants to live forever and needs to offer similar to everyone above them to achieve that? The masses?
A social contract needs most of the members to agree to it, otherwise revolution waits eagerly. So maybe a "lifetime" is available to rulers to rule, but if they want to keep living then they have to hand over to their heir at the appointed hour. Then they get a nice retirement.
So a Relict or a Med Clone would be acceptable in the case of an untimely death or critically disabling accident, but choosing to have one's thought pattern deliberately implanted in a clone to avoid the effects of old age wouldn't?