Back when I got involved in Warhammer in the late 1980s, I remember reading in the White Dwarf that after the Horus Heresy, the Imperial forces were reorganized to prevent such a large scale rebellion ever happening again.
Imperial army regiments became single type Imperial Guard regiments (e.g. only infantry, tanks or artillery) so several would need to rebel to form a combined arms force.
The Imperial Navy controlled all ships and aeronautica but lacked ground forces.
And the space marines were thus reliant upon the navy to get anywhere and use their awesomeness....
So when did the sales guys overthrow this edict?