It's an interesting semi-contradiction that the All-Hammer became mortal to build the keystone of the Divine Gate on the grounds that "what is made by the gods can be unmade by the gods, is what he said. And that something in mortal hands would serve as foundation to separate the worlds of Divine and Exandria safely, and for all good" and yet the gods remained capable of destroying the Divine Gate if the Primes unanimously decided to do so as we saw in Campaign 3.
I would assume that what the All-Hammer did is make sure that no singular god (minus the Chained Oblivion who is actually an eldritch horror lumped in together with them) could unmake the Gate.
I think, in essence, the keystone requires a hand in both the mortal world within the Gate and the wider realms beyond the Gate to be pulled and collapse the whole structure. Much like how Brennan was talking in Cooldown about how Garen's lost arm/hand was back into the more Divine realm to hold the world in place as the All-Hammer's mortal form finished forging the keystone, acting on both sides of the eventual Gate. To parallel the All-Hammer's argument for what he did: What was made by a god acting on both sides of the Divine Gate can only be unmade by a god acting on both sides of the Divine Gate. And the gods can't get inside the Divine Gate (unless they get "born" there like the Whispered One, who was immediately shoved out)
But all the gods together, united, can break the Gate because if all the other stones (the power they placed into the Gate) are pulled, well, then the Gate is gone, keystone or no. After all, a keystone alone does not make a Gate.













