Using actuarial life tables an 85-year-old US citizen has an 11% of dying within one year. Now:
Iranian life expectancy is slightly lower than that of the US
As a wealthy leader, he has access to far better healthcare options than most people on earth
While he reportedly had some health problems in the past (prostate cancer surgery in 2014 and bowel obstruction in late 2022), there does not seem to exist any active condition that threatens his life as he is regularly making public appearances
HEALTH: 2%
There is a chance that an illness will emerge and incapacitate the Supreme Leader of Iran. I'm not familiar with what the standard procedure would require in such cases, but I suspect that he would most likely remain in charge with someone else acting "ad interim". There is also a possibility that in case of severe illness, he spontaneously opts to resign his powers to a successor. Probably a very low chance, but it's still worth considering this option.
OTHER REASONS: 2%
The killing or ousting of Iran's Supreme Leader does not seem to be particularly likely. I'm not aware of the existence of dissidents or internal conflicts within the regime's political structure. I'm also skeptical about the possibility that he could be a target for foreign nations. The residual risk depends mostly on the potential explosion of new popular protests.
One further interesting point is the following.
Looking ahead, Khamenei's death will not only mark the transition to a new leader but also signify a broader shift in Iran's governance structure. While a cleric may serve as a figurehead, real decision-making power will be consolidated within a network of military and intelligence institutions operating behind the scenes. [1]*
Could the perceived shift in Iran's power structure cause some internal turmoil that ultimately topples the regime? This also seems unlikely given that the current situation favors the military elite.
DEATH: 7%
Using actuarial life tables an 85-year-old US citizen has an 11% of dying within one year. Now:
HEALTH: 2%
There is a chance that an illness will emerge and incapacitate the Supreme Leader of Iran. I'm not familiar with what the standard procedure would require in such cases, but I suspect that he would most likely remain in charge with someone else acting "ad interim". There is also a possibility that in case of severe illness, he spontaneously opts to resign his powers to a successor. Probably a very low chance, but it's still worth considering this option.
OTHER REASONS: 2%
The killing or ousting of Iran's Supreme Leader does not seem to be particularly likely. I'm not aware of the existence of dissidents or internal conflicts within the regime's political structure. I'm also skeptical about the possibility that he could be a target for foreign nations. The residual risk depends mostly on the potential explosion of new popular protests.
One further interesting point is the following.
Could the perceived shift in Iran's power structure cause some internal turmoil that ultimately topples the regime? This also seems unlikely given that the current situation favors the military elite.
[1] Iran International - Iran's Succession Decided Outside Constitutional Framework
* Slightly paraphrased