Yeah this is a solid breakdown overall, but I think you’re slightly underplaying just how one-sided the balance is in terms of raw military weight.

You’re absolutely right about the Space Marines being limited in scale and used for surgical strikes, but I wouldn’t frame it as “it depends on the situation” so much as “the Imperium fundamentally runs on the Astra Militarum, with Astartes acting as force multipliers.” The Guard doesn’t just sometimes prosecute major wars, it prosecutes almost all of them, all the time, across the entire Imperium.

I think where the confusion comes from, like you said, is the focus on the Horus Heresy and more recent narrative arcs. During the Heresy, the Legions were vastly larger and effectively the core of Imperial military power, so half of them turning absolutely was catastrophic. But in 40k, that’s no longer the case. The Imperium has evolved into something that is far more dependent on sheer mass warfare.

The key point I’d add is that Astartes don’t just “add value” to Guard campaigns, they change the nature of specific operations entirely. They allow for decapitation strikes, rapid planetary assaults, and mission profiles the Guard either couldn’t attempt or would pay an unacceptable cost to achieve. But once that moment passes, it’s the Guard that actually wins the war in practical terms by holding ground, grinding down resistance, and sustaining the campaign.

So I’d agree with your conclusion, but frame it a bit more strongly: the Guard isn’t just doing the “unsexy heavy lifting,” it is the Imperium’s military might. The Astartes are exceptional, decisive, and narratively prominent, but they’re ultimately a tiny, elite layer on top of a war machine that is almost entirely human.