Board Thread:General Discussion/@comment-31283382-20150404011045/@comment-44061-20150408005715

I think the inherent idea that immortality somehow constitutes eternal is very misleading. There like most things are varying types of immortality in relation to how said immortality is achieved. Now within the concept of bleach the Central 46 without the Sokyoku is left without any significant way of execution in this situation. Leaving them from the SS arc onward at a disadvantage. Now in regards to immortality we have to qualify that somehow.
 * Semi-Immortality/Eternal Youth: never age. (but still susceptible to damage or disease like anyone else.)
 * Object-Dependent Immortality: your immortal only so long as a specific object exists. (but if destruction comes to the object then there goes ones life or they may require constant usage of said object to maintain immortality).
 * Immortality: never age and basically recover from any physical injury. (still may have a specific weakness like decapitation or disintegration or a power that steals your immortality)
 * Unfettered Body: immortal and invulnerable. (there might be some artifacts or powers that can bypass your ability, like all the above situations you cant withstand the erasure of existence)
 * Complete Immortality: eternal and indestructible. (this is god level stuff here and i dont truly believe its applicable in this series).

So just for the sake of argument say we went by this id think Aizen would most fit somewhere between object-dependent immortality and immortality. Either which way there would be possibilities of him dying. We dont have anything to suggest that anyone including himself is really aware of the extent at which he will live cause he hasnt lived that 20,000 yrs. For all we know nigh-invulnerability or invulnerability seems to be what we are currently dealing with. As for him not being part of the cycle or birth and rebirth he was already side-stepping that being a Shinigami. Also this current arc is making clear all the genius-level intellect and science is not doing much in regards to the comprehension of gods and all-powerful beings. Aizen couldnt readily comprehend the nature of the soul king and what his attainment of his goal would ultimately mean as even Urahara had only the most basic understanding of its role. Also the Soul King doesnt cause death he maintains balance.