If we ever used them that way it might be an argument, but we don't. We always hold the line with our real armies because losing the enormous amounts of population would be too painful.
You've completely missed half the combat system. The "+80" equipment modifier is only a modifier to their battlefield skill in a sense. When you calculate that multiplier it affects the damage number you get out of the spreadsheet calculator, which is the part of the system you've neglected.
The spreadsheet that calculates the actual damage, armor penetration, armor, and hp of units. Helguard armor is hilariously higher than militia, and their weapons equally more effective. Helguard armor is high enough that militia will struggle to even hurt them. So it's entirely likely that our Helguard could defeat our entire population of militia.
The problem with arguing based on rule of cool, is that rule of cool argues for less troops. It's the reason why when we make any unit more common it loses power. Why we can't mass produce heroes or they become weaker. Militia as a whole is the definition of a mass unit.
The whole narrative of 40k is around making individuals important on a scale where they shouldn't be. Militia is against that narrative, because it's all about swarming people down under weight of numbers.
Rule of cool should mean that militia is useless. Rule of inverse ninjutsu entirely applies, the more of something there are the more useless it is. Thus militia with the absolute highest numbers should have the lowest usefulness of all.
Also saying that space combat isn't meant to be as important seems really contrary to the last arc. Not the ending, that was clearly an issue of failed balancing. Turoq's entire campaign against us was all about space combat. He relied entirely on his superior naval mobility to fuck us over and over and over. If the quest isn't supposed to focus on space combat he'd have actually landed and fought us. Instead he ran around conquering orbitals and stealing stuff from them mostly. There were only three significant enough to mention ground battles in the whole arc. Turoq's (and by extension the QM's) plan were entirely predicated on space combat. If it's not supposed to be important then the QM shouldn't have made a plan designed completely around space warfare.
You could guard a world with a 20% ratio of militia just fine. You don't need everyone in the population training 32 hours a week to form a deterrence against raids. Though really if an enemy wants to just weaken you they don't need to deal with your ground forces at all, just run around destroying your space forces, launching some bombing, and running away before you could reply would work fine.
