As someone who isn't a fan of Guilliman coming back and actually being a noble ruler (as opposed to previously the super sketchy high lords, who I thought fit a grim dark degraded tone much more), I've always considered 40k to be the bad ending. If there was a video game, the 40k we know would be an ending slide that happened when the player had truly made terrible choices. The game is over and we are stuck in the bad outcome. It won't get better.

Humanity in 40k is collectively a corpse that doesn't know or accept that it's already dead (just like the emperor himself- symbolism!). The most humanity can do are have brief moments of staving of things getting worse. Humanity can at massive cost and misery preserve a terrible status quo every now and then and call it a victory, but even the victories just lead to losses and the losses keep leading to the final death of humanity.