: titans leave well of eternity behind
We're not even sure if blizzard still remembers this given their constant chronology changes in the old god story.
It's still "Canon" but given blizzards track record they'll prob retconn it.
+ The mission of the dragons is fulfilled. It makes no sense that this mission was to kill one of their own, so it looks more plausible that this mission was to learn how to do stuff on their own (thrall replaces neltharion for a time, kalec is made aspect, ysera, nozdormu and alextrasza resist corruptio
I wouldn't try to make sense of it, It's based on terrible writing, time travel plotlines and lack of explanation. At best we can assume that they sacrificed their "Heart" gifts to fuel the final attack from the dragonsoul.. one capable of unmaking deathwing.. Or you can take the "destiny route" and attempt to make that concept sound less mentally impaired.
The question is: why this mission?
Again, blizzard implying a "mission" or a "destiny" is pretty bad on the lore part. The aspects were charged with maintaining a world that was forced into order, Their charges cover almost every threat to the terraformed world when combined with Titan machinery and seed races.
Their jobs can not just "End" Unless every druid covers the Greens and Reds, every mage cover the blue and bronze and every shaman cover the Blacks the planet will collapse on itself. Even so the aspects loss of immortality never said that the dragonflight lost their powers.. but if they all did then blizzard has really messed up.
corruption was there before the burning legion, and Sargeras has fallen, then commanded the Legion
Legion corruption is a "different kind of high" than old god corruption. Legion corruption is basically giving you an addictive drug that enhances you to the point where you feel like a god. It doesn't make you instantly evil by force but would make you evil out of your own greed and pride for power. Sargaras could have been "Corrupted" by the nathrazim but chances are he joined based off his own rationalization and philosophy. This is why there are demons not affiliated with the legion.. such as the demons that teamed up with Illidain or Legion deserters like Sal'salabim.(who allowed to stay in the lower city and drink, showing that it's more of a hatred for the legion itself then just demons)
Old god corruption is essentially mind molestation that ends up mutating and twisting your body over time, Lovecraft stuff essentially.
Although sargeras approach on hell was a bit more fiery, titan corpses could "evolve" into old gods, spreading the undoing with the same power they created life. These "sons of cthulu" needed a place to be confined, and guardians to keep them quiet. Is that why the Titans shaped azeroth in the first place, and put there people who would do everything in their power (and a wheel of magic to power them) to keep the cemetery door closed ?
From what we know Titans are giant metallic men..not THAT far from their own creations which could hint that the titans themselves are artificial, Heck I have a crack theory that the titans are small organic race that uses mecha bodies... but besides that.. Titans are not gods.. in the sense of the warcraft definition of a state of being that is shared by the incomprehensible old gods and Elune.
from what we've learned in their ancient cities is that they use incredibly advanced technology and are very skilled with science in almost every field.. from a powerful understanding of living things and elementals too the tricks of Arcane/ Timespace and matter.
They seem like gods, but they aren't, just a very advanced race.
That being said they wouldn't degrade into old gods upon corruption...