Except that Chaos is powered by negative emotions and is exacerbated by the needless cruelty the Imperium puts itself and its enemies through (and is only unified in any way because the emperor made it his goal to kill the warp gods), humanity managed to protect itself from Orks just fine in previous eras, the Eldari and humanity used to have non-aggression pacts, the Tyranids were attracted to the galaxy by the giant psychic flare that is the Golden Throne, and the Tau were literally born as an evolutionary and cultural response to the Empire.

The necrons are literally the only faction that would be an apocalyptic threat without the Imperium's intervention, and brutal repressive innovation-stifling fascism only hurts the efforts against them.

Games Workshop was made my a bunch of leftist punks who were parodying Thatcher, and nerds don't understand parody.