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.
Yeah, that's exactly why the Imperium should embrace Chaos. Chaos has been advocating for the survival of the Imperium for a long time and are actually the good guys. They got so close with the Horus Heresy but the Imperium, being the conservatives they are, decided that it wouldn't be the best.
Chaos will give the Empire the power it needs and usher in a new golden age of prosperity and will bring all those filthy Xenos to heel. The Empire is dying, but for it to live the Emperor needs to die. Death to the false emperor. ☀️
Well if you go by the predictions of the Cabal, 40k is the bad ending at least when it pertains to the galaxy as a whole. Chaos slowly grows feeding on the corruption and unending war that humanity brings while none of the species truly flourish (except for maybe the orks and the tyranids).
Whether it is the good ending when it pertains to humanity only is up for debate because there are several hints dropped in 30k that this was only outcome which ensured humanities survival
I actually like Papa Blueberry getting to wake up and say "What the fuck" to really emphasize that even the ancient space crusader thinks this future is FUCKED, but I have no faith in Gee Dubs to go anywhere worthwhile with it.