Yeah, my issue isn't that ground troops aren't a necessary component of Imperium ops. It's that ortillery changes the character of the wars they will tend to fight, skewing them towards "low-intensity" counter-insurgency ops, often with a political dimension. For Army and Marine characters, that means the military experience is much less operationally (and ethically) clear-cut than I might otherwise have imagined. Especially if they happen to be on the losing side of the space war.
Well, at the 30000 ft view, all wars have a political dimension. The detail is simply that Back In The Day, The Body Politic tended to be rather isolated from the battles, if the Body Politic really had any say in the campaign whatsoever.
At the military level, (let me get my black and white paint brush out here), it's Duty and Mission. "Politics are above my pay grade", etc. Rather, adhere to the Rules of Engagement, perform the mission, and get home with your buddies in one piece.
All that said, I do not think Ortillery changes the battlefield dynamic as much as you think it does. It's "simply" air power. We had a crapload of air power in every engagement from WWII on. But, no matter how much you have, it's still a limited resource, and in all cases, it's more limited than the ground forces.
Now, things like Grav Tanks can muddy the argument. They're effectively NOE skimming, armored air power. But when the fleet arrives with the equipment, you're going to have a lot more shorter range but equally effective gun barrels on the "ground" than you are on the ships. Off shore artillery was certainly useful in WWII, but it was still artillery, and expensive artillery at that. Better to make a hole, make a beach head, and land more "cheap" artillery than rely on the expensive stuff floating in orbit.
You also have the issue of cost of loss of the unit. It's "cheaper" to lose a crew served, autonomous weapon than a larger platform. Harder to kill 10 tanks with fusion guns, than a ship with 10 fusion mounts, plus they're more flexible to deploy, and take damage at a more granular level than a ship. Also a lot cheaper to man.
Give me a 1 or 2 dozen meson sleds with a 500 klick range to one spinal mount with a 1Mkm range any day of the week.
And it's probably obvious that the Iraqi theatre has influenced my analysis of 3I warfare. The asymmetry of force between the US military and the insurgents seems like a good RL analogy for the asymmetry of force experienced by an invaded planet with battle-cruisers in orbit. Especially when I think about whartung's comment about Ground CAP.
But also recall the asymmetry of force between the Afghans and the Russians. American Stingers effectively paralyzed the Russian Campaign. When you have an air-mobile doctrine, without air, you're not very mobile. When you have the Aghans blowing the roads out from underneath your armor, and dropping them in to canyons, your columns stall and you can no longer bring the fight to them.
The Afghans leveraged their terrain VERY well, and the Stingers worked to offset the one advantage the Soviets had. We're having a frustrating enough time as it is now, and it's mostly the terrain we're fighting as well.
Basically, what changes the game is not the technology, it's the rules of engagement. And that's ALL political. Things like being careful of civilians, avoiding culturally sensitive installations.
The orbital units can provide intelligence, and quick response firepower. But that firepower doesn't have the fine resolution that ground forces have.
If you just want to pound the opposition in to paste, i.e. a very loose set of rules of engagment, then ortillery may offer some advantage with high powered energy weapons or deadfall ordnance. Large meson guns obliterating 100 yard radius circles at a time can have quite an effect on an enemy.
But when the rules call for more finesse, their usefulness starts becoming more limited.