[Stupulous]

If people live forever except for those in power, and power is opt-in rather than, e.g., sortition, power can only go to people who prefer it over eternal life.

That group will be composed of the insane/irrational, those who value power at extremes, those who value improving the world over their own lives, and those who have lived long enough that they're ready to move on. The last two categories are great candidates for empowerment, but one is rare and grows rarer, and the other doesn't show up for upwards of centuries. Until then you mostly get the first two, which suck. Eventually, I think, we burn through everyone willing to make the trade-off and we're left with no power centralization.

Unless you mean they only face death for the duration that they are in power, as an enforcement of term limits. I don't think that is materially different from other enforcement of term limits- if a dictator is able to compromise the military, they can probably also compromise whoever has the power to stop their impending death.