Related thought: the 13th Black Crusade would have been dealt with a lot easier if they hadn't shipped approximately eleventy bajillion dudes off of Cadia across the galaxy. Hell, given how often Cadians turn up they probably could have kept them all there and launched a successful crusade into the eye
I wondered that, too. I mean, I know that the Cadians have become something like the Ultramarines of the Guard, and that they are really tough even on basic troop level, so you'd want them around, but really? Cadia, the most defended place aside from Holy Terra, where they make a point of making everything both a fortress and a weapon, a world that is constantly threatened by literal hell? Let's draw some soldiers from there!