Okay, so we have to break this all down. Zeus, by providing Pandora is enabling a method for humanity to learn and grow past their mistakes. Sure, they have to suffer through some stuff to do so, but he also threw in hope to make sure it was possible to grow.
Zeus goes on to father heroes, providing examples for what humanity should aspire to, what they have the potential to become, whether it’s Perseus using his wits and Batman-esque items to save the day and rescuing Andromeda from her blaspheming parents or Hercules setting aside his own pride to atone for actions under madness.
Lastly, we have the realization of the human potential in heroes such as Diomedes and Odysseus, representing physical and mental prowess of humanity, respectively. The question we’re faced with, though, is why does Zeus want this?
We’re on shakier ground, here. Zeus, being Zeus, never comes right out and states his purpose. We’re going to have to infer his purpose from the evidence at hand. The actions are too deliberate to think it’s accidental. We know that he wants humanity to grow.
Furthermore, we know that Zeus is something of a politician. He can make alliances, and he doesn’t renege on them. He was honest in his dealings with the Cyclopes and Hundred Handed-ones.
He forgave the Olympians for their attempted coup, asking only an oath from them.
He is adaptable. He can adjust what he’s doing and change his own behaviors. He is a fierce opponent resorting to brutality when he feels it’s justified, will avoid entanglements in his mediations between the gods, even relying on his mother Rhea (yes, the one he raped) to mediate between Demeter and Hades.
Out of all the Greek gods, he is the most skillful and adaptable, mostly because the others, as we’ have seen, are incapable of change in any way. They are the way they are. Forever.
But with humanity, there is a race that can change and grow, and they have done so at a phenomenal pace, cosmically speaking. In a few dozen generations, human heroes have shown that they can challenge titans (Odysseus defeating Polyphemus) and the Olympian gods (Diomedes routing Ares). It’s only a matter of time until they will be able to rise up as an entire people to become more powerful than the gods themselves.
The only gods they will be able to replace, though, are the gods who cannot change, meaning the rest of the Olympian gods. Zeus himself, however, will be immune to them as he will continue to grow and change even as the people do. Moreover, he has been their patron from the beginning, skillfully manipulating events to most benefit them without the other gods ever knowing.
Could they ever rise up in power enough to challenge him? Well, sure. But then, that’s just the pattern established since the beginning of time in Greek mythology. The son replaces the father. Zeus, however, will not be replaced by the likes of the unchanging Olympians, who have proven themselves spiteful and petty in their disputes. The god who eventually replaces Zeus will be a worthy successor capable of the same growth as Zeus himself, if there’s ever even a need to replace such a skillful and capable king of the Gods.