Tyrannids, Orks, Black Crusade...
I have a few more, But I disagree with the "We don't need to mobilize" bit. There are a lot of threats.
Orks are a fair point, but they don't require the
gross level of mobilization that the Imperial Guard represents literally every minute of every day--and Chaos tends to get most of their hulls by just straight up stealing it from vulnerable Imperials who go rogue and take their ships with them.
And the Tyranids literally
just showed up. They're exactly the kind of threat that you maintain a reserve for--so you
can dial up to meet the new needs, rather than constantly running the engine on overdrive and just trying to outright replace parts of it in use.
The Imperial Guard is as poorly equipped and numerous as they are because their
primary objective is the suppression of local rebellions. Because they're still better equipped than all but the most seasoned PDF forces, and that's the only reason they need such enormous
mass too. A tighter, better equipped elite infantry cadre supporting strong PDF forces (Rather than ones that have their best cadre literally decimated as a matter of course about twice a generation if they're
lucky) would mean that the Imperium would lose planets
very rarely, and generally only against threats that are
worthy of dispatching elite forces against.
But it also means the Imperium loses the ability to keep their individual planets weak and dependent on them. Which was the policy of the Crusade, because they needed
literally everything they could pull out of every world in order to keep up with the absurd tempo, and nobody ever told them to
stop despite the fact that working more like a federation where everyone contributes a modest--but
reasonable portion of their economy to a central pool, while having enough to see to their own affairs without needing to be shackled to the central authorities--would be a superior survival strategy.
So they kept doing it, even when it stopped being a good idea, and for all the Imperium's disgusting amount of resources, they end up having to waste most of it putting out the fires that were created strictly because of the stresses brought on by nigh-permanent full mobilization.
