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My toxic trait is I think Star Wars could absolutely defeat Warhammer 40K, I don't know a lot about 40K but from what I have gathered their planet destroying technology is kinda crap, like they can destroy the surface and they can only destroy the whole planet in steps.
Star Wars has the Death Star, which destroys a whole planet, and they can create multiple of these, it also has the Starkiller which can destroy 5 planets from solar systems away, it also has the New Order Fleet, which counts I guess, its stupid but its technically cannon and its absolutely ridiculous. And if we count Legends we have the Sun Crusher, which literally destroys entire solar systems, and there are no downsides or internal backlash to destroying planets in another Galaxy.
The Star Wars Galaxy has also been canonically unified more often then not, either a Republic or an Empire is like five times larger then any one faction in 40K, which is a hopelessly disunified Galaxy, the Separatists alone had quadrillions of droids, the trillions of soldiers of the Imperium is a drop in the bucket to that.
So what I am imagining is the Star Wars Galaxy nuking the shit out of 40K and perhaps allying with something like the Tau, and repelling any attack from the disunified factions with overwhelming numbers of incorruptible droids.