But it would make an great novel though...And give the Alphas an actual purpose than just staying in the dark, scheming all day and nothing much to show for it.
Stagnation is one thing that the series is known for.
I personally favor the theory that the reason the Alpha Legion tends not to get much done is because they don't even have a cohesive plan anymore, just thousands of warbands and solo infiltrators and spy cells scattered across the galaxy, all of them doing their own thing in an effort to accomplish a variety of goals that they may not even remember anymore, such as this list from 1d4chan:
1. Those who are actually still loyal and aid the Imperium as best it can in sneaky, inscrutable ways.
2. Those who are nominally loyal and uncorrupted but still carrying out the Cabal's plan to make the Imperium lose.
3. Those who have embraced Chaos to bring about the other version of the above plan.
4. Those who have embraced Chaos in a misguided effort to bring about the original version of the above plan.
5. Those who have embraced Chaos and are just doing their own thing. Sindri Myr comes to mind.
6. Those who are loyal only to the distant memory and legacy of their Primarch (either one) and Legion and act in whatever way they think best preserves that legacy.
7. Those who are loyal to the Emperor but deeply oppose the current state of the Imperium.
8. Those who are loyal to the Emperor and think that by destroying the weakest parts of the Imperium they strengthen the Imperium as a whole. Occam the Untrue specifically believes this in
Sons of the Hydra.
9. Those who care only about the survival of their warband and brothers, even if they venerate Chaos/The Emperor/The Primarchs/etc. Kassar from
Shroud of Night falls in this category.
10. Those who do their own thing, just in general.
11. Those loyal to their own conception of the Alpha Legion. Whatever the hell that entails. Which is most of them.
12. Those who are motivated by their hatred for something in specific, usually Guilliman. Quetzal Carthach from
The Long Games at Carcharias and
Sons of the Hydra just really fucking hates Ultramarines, allegedly because he was at Eskrador when Guilliman killed “Alpharius”.
13. About a million other things.
14. Fuck knows.