Board Thread:Warhammer 40k General Discussion/@comment-14745711-20140928092626/@comment-25451195-20140929035608

Neithan02 wrote: primarchs would split the imperium if they demanded power

and they would not want to be overlords of just 1 chapter

Primarchs were born to be... Princes of the Universe (Galaxy in this case), power rests on the strongest brow, and their's were the strongest, and all the successor chapters of that primarch would want to serve under their Genetic origin, so all the UltraMarines Spawn would come together if RG came back, same with the DA in LelJ's case.

Back in 3rd and 4th there was always hints that the INQ was there to also maintain status quo, and the Lords of Terra want things to stay the way they are. Any Primarch of the loyalist side returning to the Galaxy would shake things up, and start a civil war in a heart beat.

there is a Space Wolves novel with a statement about Terra in it....
 * "Beautiful on the surface, but rotten underneath. Don't ever, even for a second, doubt that this is the most dangerous world in the galaxy.... Danger does not always come in the shape of Orks with bolters, Ragnar. This world is where the elite of the Imperium have gathered. We are talking now of the most ruthless, ambitious, unscrupulous collection of rogues ever culled from a million planets. This is the place they have come to realise their ambitions, and on Terra they can, and will not let anything stand in their way. Not me, not you, not their own kin if need be."
 * — Torin the Wayfarer, Wolfblade to House Belisarius of the Navis Nobilite, speaking to Ragnar Blackmane