DISCLAIMER: i don't know if anyone else has proposed this theroy or anything like it. If they did, link it in the comments.
At the beginning of God of War 3, you fall into the River Styx after being attacked by Zeus, and later after defeating Hades. In Greek myth, Achilles' immortality and invincibility comes from his mother dipping him in the river Styx as a child, but as she had to hold him by his heel, he wasn't fully protected. Unlike Achilles, Kratos was fully immersed multiple times.
In canon, Kratos hasn't died since the beginning of God of War 2. We see in the first fight against Baldur that Kratos has a regenerative power. In the novelization, it is stated that 50 years has passed since God of War 3, although Kratos looks more like 45 – 55 years old, not 90 ish years old. This could be attributed to his immortality.
All this being said, the end of God of War 3 seems to suggest immortality, where as the PS4 God of War suggest extended longevity like Aragorn and the Dunedain from the Lord of the Rings, and a healing factor, though it's not clear if it's voluntary or not. I do not think this is only his Godhood at play, as although it is unclear whether or not he ever gains his Godly powers back from The Blade of Olympus (that's another theroy in itself that may explain his immortality), he always maintains some powers, mainly his physical Strength, however even with his Godly power, I believe he shouldn't have survived his suicide attempt. The soul of Hades may have helped or been the deciding factor, as Hades has Regenerative powers, as well as being a God.
Let me know what you think, and even if you disagree, comment and upvote to keep this discussion going, and maybe get the attention of Cory Barlog, the rest of Sony Santa Monica, and if we don't get an answer from them, maybe MattPat can look into this, since I'm pretty certain he hasn't made a God of War Game Theroy.
© Post "Theroy: Why Kratos survived God of War 3" for game God of War.
Top 10 Most Anticipated Video Games of 2020
Top 15 NEW Games of 2020 [FIRST HALF]