I just realized that Constantine isn't supposed to be WH40K's equivalent to Augustus... He's Constantine.

Undefeated, but ultimately still the only thing holding a dying empire together. He is the empire's last breath, last hope of survival. But when, not if, he dies, there will be no one left to hold the pieces of this empire together.

As Constantine shirked the old pagan religion of Rome in favor of the persecuted Christianity, Guilliman has shaken off the millennia long brutal enforced monotheism worship of the Emperor of the Imperial Cult, and though he's now leading the Imperium, crushing all enemies who stand in its way, the only thing causing these victories is Guilliman. All he's doing is delaying the inevitable, just as Constantine continuously stopped the Germanic incursions into Rome, toppling all challengers to his position as emperor of Rome, and prevented the collapse of the Roman Empire, so too is Guilliman doing the same for the Imperium.

And yet, just like Constantine, the Imperium is doomed to fall when Guilliman dies. But in this instance, it's a question of if there's anyone worthy to replace him.