Board Thread:Warhammer 40k General Discussion/@comment-14745711-20130906234048/@comment-24944355-20141119230415

I didn't say things wouldn't change only that Sanguinious is the only one who could unite all the empires factions behind him & enable at least a part of the imperium to survive in some form following the Emperor's almost certain transformation into a warp god.

None of the other primarchs, not even Guilliman, would have enough unwavering support from all imperial factions to pull it off. Ultimately without Sanguinious the empire shatters completely, with him at least a large portion of the imperium will hold together.