Life in itself is worthless. The only reason why life is valuable is because it is a canvas, a vessel through which you can find happiness, meaning and so on.
This means that if you have a life that isn't worth living, you are not at risk of losing anything of value, and so the potential loss is still bounded. Sure, there is the potential that life can change in the future, but whether you have 10, 100 or 1000 years left of potential life, you don't really care much about that if your life is an agonizing living hell.
Sure, lots of philosophers talk about lives barely worth living, and what constitutes the line. Lots of public health researchers work on metrics for quality-adjusted life years, and increasing longevity is useful only insofar as it increases QALY's.
But I really do expect most measures which increase population longevity to increase population QALY's. The conflict between hypothetical immortal tyrants and immortal coup-conspiracies would only be a small part of this; material conditions and overall societal wealth would weigh much heavier on the scale.
This means that if you have a life that isn't worth living, you are not at risk of losing anything of value, and so the potential loss is still bounded. Sure, there is the potential that life can change in the future, but whether you have 10, 100 or 1000 years left of potential life, you don't really care much about that if your life is an agonizing living hell.
Don't ask me how I know.