When an Emperor dies or is born, the impact on the world is seismic.
An Admiral dying's barely a blip on the radar.
There's a reason it was the Three Great Powers - those lightweights can't carry justice on their own backs. They've had to be supplemented by Warlords, Pacifista and now Seraphims because they're small fry to Emperors, who are infinitely more threatened by each other.
An Emperor versus an Admiral is the equivalent of a parent wrestling with their kid.