Well for instance, how much power the Inquisition has gained throughout the years. I just feel that realistically the High Lords and Guilliman's allies would remain the strongest and most influential force in the Imperium.
The strength of the Inquisition stems from its decentralized nature. The High Lords just get most day-to-day stuff too late to do anything about it. Even stuff like Armageddon or something just takes too long for it to reach the ears of the High Lords, through all the channels of bureaucracy it normally has to wind through, then they have the deliberate, and decide on a course of action, and then finally shoot the answer back.
The Inquisition is at ground zero, usually.
Plus there's just so much momentum behind the Inquisition. People "know" the Emperor is a god. They also know that the Inquisition also speaks for the one true "god". How else are people supposed to react? It's so ingrained in the peoples' mind that the Emperor is a literal god, that to go against the physical manifestation of His will would be insane.
As far as Guilliman goes, I'm quite surprised he is able to gain enough support and strength to essentially force the Imperium to his will.
Remember, it wasn't just Guilliman giving the a-ok. Corax and the Khan also sided with Guilliman.
Guilliman was also the only primarch to actually build an incredibly prosperous and productive empire. That was probably a giant checkmark in his favor.
It also made sense for those who
opposed Guilliman to buy into the Codex Astartes. Who had the the greatest concentration of Space Marines in post-Scouring Imperium? Guilliman. Who had the most to lose by breaking the Legions into Chapters? Guilliman.
I would think there would be some sort of understanding that they were all against Chaos together.
I think Guilliman was fully aware that either they stuck together or died. That's why he made the Codex.
It wasn't "you're with me or against me!" but more "we need to work together without killing each other, so here's how we're going to do it..."
At least in my opinion.