Let's do some maths, ok?
Pre-Heresy: Ultramarines Legion - 250,000 Astartes.
Most valuable Allies: 1,400,000 Astartes from other Legions (not counting Astra Militarum)
Most valuable enmies: Orks.
Note: Emperor forces has the most advanced and lethal wargeag EVER.
Post Heresy:
We all know...right?
Soooo:
Roboute: "Legions are too strong! Because never again such power should be in the hands on one!"
- "You are right Bro! So, what do you think? Should we split them in half? cut them into ten times smaller groups?"
Roboute: Negative. Split them in groups 250 times smaller!
- "............You mad bro? "
Really. I understand the reason of the Codex Astartes, but apart from rules and directives, the micro scattering of the Astartes is so wrong on almost every possible level!
1) Now you faces Chaos Astartes. Did someone tells Roboute that Chaos Space Marines don't follow his Codex, so they can freely decide to have numbers on their side almost 100% of the times.
2) You Lost that super efficent tech and wargear you once possessed. More, actual enemies are not only Orks and the new Chaos SM, but Necrons and Nids. They are all not famous to move in small groups, nor they have particulr difficulties in bolster their ranks numbers.
3) Often more than one Space Marine Chapter needs help from other Chapters to succesfully faces those threats, but they lose their unity, trust, and experience in fighting as a group. To the point that some Chapters looks each other with doubts and supects about loyalty to the Empi.
I have other points to say, but i think it's those three are enough to make my point.
1000 Astartes per Chapter is a joke.
Roboute went way too far, probably for the shock of a wounded and dying Empi, probably for the loss of some loyal brother, probably for the shock to see Astartes killing Astartes, probably to see Primarchs embrace Chaos. Probably for all those things together. But he went too far for sure.
You have 700k Astartes that turns from super valuable allies into super dangerous enemies.
You have to face chaos, corruption and distrust to the Imperium of Man and his Empi.
You have to face old known enemies like Orks and Eldars, and new ones (very scary ones) like Hive Fleets and Necrons.
How gropus of 1k Astartes, scattered from each other, alone, with worse wargear than ever, in a diyng empire with a dying Emperor could be the solution?
I'm pretty sure that if Roboute will recover from his wounds, the first thing he has to do is trying to kick himself in the butt at first, and then make a second Codex Astartes where organization should be done keeping in mind the actual situation, and not only the shame and the horror that utterly devastate the Legions of the Emperor, made by the Empeor, wanted by the Emperor, and most important Legions that worked with precision and unity for millenia, before the Horus Heresy.