As the Terrans, I am in a war with the Drengin. I conquer a planet. Billions die. I conquer another planet. Billions die. That planet is reconquered by the Drengin. Billions die.
The gameplay effect of this massive tragedy is a gaping "meh." Sure, my production dips a little on the planets contributing the soldiers, but that's not exactly the kind of thing you feel in your gut.
The deaths of billions, particularly when you lost a planet, should be something that hits you in the gut.
EUIV has an excellent war weariness mechanic that reflects the crushing effect that lengthy and costly wars have on nation-states. I am not suggesting that Galciv could or should "cut-and-paste" that mechanic, but some sort of ticking global malus would really add a much-needed cost to waging vast intergalactic war. It would help me suspend my disbelief a lot more than the current sterile mechanics.