Ship of Theseus issue:
a soldier on a TL14 battlefield has half their brain destroyed, but advanced medical science stabilizes them and them the half a brain is regrown.
Are you still the same soldier, although lacking personality traits and memories?
Say a wafer back up was taken before the conflict and you can be resored to that point - are you still you?
Another battle, this time the original half of the brain is destroyed. You get your brain regrown again - you now have no original brain (by the way - did you know you don't have the brain tissue you were born with?) and yet full memories and personality are restored.
Are you still you?
How about the entire brain is destroyed but the rest of the body survives - the brain is regrown and again a backup implanted. Is it still you.
Of course I don't have "the" answers to these questions, assuming in fact there even is such a thing. But I'm also actively avoiding philosophical discussion to prevent Pitting.
There is something to be said for the fact that, despite the availability of these technologies, there's no evidence that emperors use them. Of course, we actually don't know what happens to many of them, and it could be that the title passes along some kind of rule following the "original" lifespan.
But the most likely answer is that come restraint prevents its use among the highest echelons of society. And hence the entire point of my original posting of the topic.
What are these?
1. Technical: It's possible that continuous life over centuries is simply not possible (at least by 1100). Perhaps psychological issues creep up. Or there's some kind of degradation over time that eventually prevents transference.
2. Social/Cultural/Religious/Legal: It may also be the case that immortality is technically feasible, but rejected due to social, cultural, religious or legal restraints that are sufficiently powerful so that nobody at the top tries it.
We know these conventions are not universal, because "death insurance" does exist, and the Imperium is huge and heterogeneous, to say nothing of other interstellar regions.
But it could be that they are effective in curtailing any such attempt by the emperors and senior statesmen.
Although, we actually don't know that all the previous emperors have "died," just that the throne passed. Some of them could be milling about as emperors emeritus, but that seems unlikely. Especially since in some cases the throne passed violently, and one's usurper is unlikely to leave the legitimate ruler lounging around.
3. Personal: It may be that people who embark upon an attempt at immortality fail, for various reasons, and after a certain period of time, elect to "check out." So while there's no technical or legal restraint, people just don't live forever.
And of course this list is not exhaustive, there could very well be other reasons.