Hey remember that time where Games Workshop tried selling the entire Ultramarines chapter in one box?
Jeez... ...how long would that take to paint.
Sometimes it seems like any "bad guy" faction winning would mean extinction for mankind.
Actually, I dunno any more, reading Codex: Necrons (3rd Edition) closely. As long as they kept humanity alive as slaves, a C'tan/Necron-ruled galaxy was the least worst horrific victory, out of all the existential evil factions; the C'tan/Necrons, the Orks, the Tyranids and Chaos. Under the C'tan/Necrons most humans would survive as slaves or subjects to the C'tan Empire, live on a million Necron worlds, persisting for the rest of time, perhaps having some measure of a normal life, playing in the barren wind-swept streets as rag-clad helots under the eyes of their silver masters, before one's probable sacrifice, say age 30-50, in a batch of 100,000 victims being coralled by gauss rifles into a temple to the Star-Gods. C'tan would actually encourage humanity's survival as a species, as sacrifical slaves (unless they fucked up their maths, ate too much, killed everyone, then went to bed for 60 million years while more sentient food evolved).
Orks would presumably just reduce humanity down to roving Mad Max bands, like the Diggas on Gorkamorka, but most humans would die of collapsing infrastructure and lack of food. It would entail the death of most of humanity. Life would be a harsh battle for survival. Orks lack the motivation or studiousness to kill all of humanity. They just wreak shit; human civilization would end.
Perhaps Chaos is next "least worst", as stupid as that sounds? Some people believe a total Chaos victory, would just destroy reality, or break the barrier between reality and the Warp at least. Rape worlds with faces growing out of every surface kinda thing. However Chaos Space Marines specifically want to overthrow the God Emperor and remake the Imperium as a polytheist Dark Imperium. Take it at face value, that it wouldn't just devolve into murderfuck (destroying reality), and assume that institutions would keep it a functioning Chaos Space Marine empire with central government. Then it looks more like an extremely dark social Darwinist dystopia, where humanity survives, but a few people grow an extra leg or three, and betray their relatives for extra eyes on their elbow.
All the buildings would get more spikes, you would be taught to sacrifice puppies at school, be taught survival of the fittest from Chaos kindergarten, kill some classmates playing Blood Bowl in year 7, get randomly stabbed by a passing Chaos Space Marine visiting the supermarket, and a lucky 0.00001% would become Chaos Champions in the Premiere League for Tzeentch FC vs. Khorne United. Maybe.
The thing that makes Chaos hard to comprehend as appealing, is that possession means being a mere meat puppet, and complete Chaos reality-breakdown means either eternal body rape or incomprehensible caledaiscopic soul rape. That is where the motive breaks down, if Word Bearers say, actually know this is the outcome. To make Chaos more comprehensible, I now prefer to imagine that a Chaos Space Marine total victory, turns the galaxy into something like a stable Dark Imperium of Man, with Abaddon the Despoiler sitting at the top of a working civilizstion, as a Demon-God-Emperor.
Temples to the Chaos Gods are erected on every world, the galaxy kept in line by Chaos Space Marine legions. Humans are encouraged to become completely amoral, compete with each other to be champions of Chaos, but never go to the extreme of birthing some new Chaos god accidentally; the way religion often lies atop a society, but the majority just persist without exceptionally strong convictions. Survival of the fittest, absolute rule of the strong, but survival of human life in a Dark Imperium, somewhat like the Chaos route in Shin Megami Tensei: Strange Journey, where humanity is totally "liberated", but in practice the demons, the strong, dominate the weak, meaning worse tyranny as 99.999% of humanity is unlucky or weak.
This makes the Tyranid total victory arguably the worst, since it just entails complete annihilation of life itself, the galaxy stripped bare of even microbes. At least humanity survives in the Necron, Ork, or even Chaos scenario, with Chaos probably being the worst of the them by fa. Mass human sacrifice, forever, under a C'tan/Necron Empire, at least doesn't involve the anarchy and torture of a Chaos Imperium. You just live in eternal poverty, able to somewhat live life, under your Necron lords, until the day of your sacrificial murder.