So I just got here, and this is crazy,
But the primarchs seemed pretty useless in the absence of their Emperor.
So just make new ones... maybe?
At this point I would imagine there are far more qualified people in the woodwork, just waiting for a chance to actually lead and/or make a difference. Unless Guilliman is a flat out astronomical genius, then honestly he seems irrelevant. He'd have 10,000 years of catching up to do, and his Ultramarines would be but one chapter, give or take those offshoots that are intensely loyal. The bureaucracy he helped put into place would no doubt buck his every initiative, and if pressed, could cause another civil war.
"There are old heretics and bold heretics, but there are no old, bold heretics. How do I know? Because I make sure of it myself."
Wing Commander Silo Barstad, Imperial Navy 3355th Fighter Wing