Note that infantry backed up by heavy weapons has historically pretty much always been the most industrially efficient military force, with few exceptions. And in those cases a counter was devised that resulted in the supremacy returning to properly equipped and supported infantry forces.
This did not change with the invention of the gun; if anything, as the gun became more effective the infantry became more potent. Even today infantry has a key role in warfare, and one of their tasks has been one that has occurred throughout history, fixing the enemy in place so that harder hitting weapons can be used.
That said, there are a number of ways in which infantry actually is definitely superior to more industrial power demanding units. For one, the fortification mechanics greatly favours infantry forces, to the point that a properly fortified infantry unit is very difficult to dig out of mountainous or overgrown terrain. And the gods help you if the terrain is both. Two, infantry does in fact do a better job policing a planet then tanks do; enforcement here is dependent on numbers, not the biggest guns, so an infantry unit with the greater numbers would be much more capable in this job and quite possibly cheaper than a force of tanks with the same job. Three, infantry just lets you stack a lot more weapons on the same load of tonnage. They're not as sturdy as a vehicle, but you can generally expect to stack at least 2 weapons for the same amount of tonnage as with even light vehicles.
They also have a few drawbacks. For one, infantry can't carry Medium or Heavy weapons. For another, when hit they are extremely fragile. And third, CAP and HCAP weapons will mow infantry forces down in a way no weapon system can mow down vehicles or static units. Their volume of fire is obscene compared to other weapons.
Raw manpower isn't an issue in Aurora because in Aurora the ability to equip and ship soldiers is insignificant compared to the total population of a given empire. A major ground side engagement would be at most a few divisions slugging it out with maybe a few hundred thousand soldiers total, when the nations involved both number in the billions in population. At that point, even if everybody dies over the course of the fighting it's a rounding error compared to the total number of people that die every year. Trained manpower is an issue however, because there's only so many academies and GFTFs you've got running that can get you the personnel you need.