2 Things.
1. Aside from the fact the world was destroyed I know next to nothing about AoS. I do know for a fact that 40K galaxy wasn't destroyed so I don't know where you're going with this.
2. We're talking 40K not 40 so let's not go off on a tangent shall we?
http://warhammer40k.wikia.com/wiki/Great_Rift
So maybe I was a bit off but it's hinted that the breaking of Cadia (Which Abaddon pioneered) was the "breaking point" which caused a massive warp rift that cut the Imperium in half.
So essentially that means that communication with the dark side of the Imperium is next to impossible and they are more or less cut off.
The necrons are back on the offensive.
The Tyranids are closing in on Baal and the Angels have zero reinforcements.
The Damcoles Gulf fire has been extinguished so the Tau are now back on the offensive with another massive expansion fleet.
On the far side of the galaxy-spanning Warp rift from Holy Terra, things have quite literally gone to Hell. There, the light of the Astronomican is obscured behind a psychic maelstrom of nightmares and the entire region has been dubbed the Imperium Nihilus, or the Dark Imperium. Amongst the besieged Imperial worlds in this Dark Imperium are many Space Marine Chapter Planets, including Baal, home of the Blood Angels. Things were already looking grim for the sons of Sanguinius, with a Tyranid Hive Fleet closing in on them, and now it looks even bleaker. Cut off from the light of Terra, the Blood Angels and their Successors stand unbowed against the might of Hive Fleet Leviathan, hoping they can withstand the storm.
So I fail to see how things are "noblebright". Sure a loyalist Primarch has returned but the situation is pretty desperate for the Imperium.
But the scale of Guilliman's task is monumental. Following the emergence of the Cicatrix Maledictum, no world in the Imperium has been untouched by war, and there are many thousands of planets that cry out for aid and reinforcement.
There is no way Guilliman will be able to save everyone even if he could.