We already know the Emperor could do better than the regular marines and even the Primaris marines, because not only do the Primarchs exist, but Custodes and Thunder Warriors do as well.
People who make comments complaining about Guilliman and Cawl outdoing the Emperor simply never think critically about their complaints. Too many people on the Internet fail to use any mote of logic or reason or analyse their ideas before flinging them out into the void of the web.
I respectfully disagree with you.
The Thunder warriors are only 'better' if you take the narrow-minded view that just because they were bigger and stronger that it made them better soldiers. If they had been capable of conquering the galaxy like the Astartes did then the Space Marines would not exist, people often forget that the Astartes are exceptional in mind as well as body. Similarly, the Custodes were created as the Emperors personal bodyguard, each was a bespoke creation which represented a significant investment which prohibited them from ever being produced on the same scale as the Astartes (not that that was ever the Emperors intention).
Most (if not all) of the arguments I've seen so far have been well constructed and thought out. The problem is that you are trying to apply 21st-century logic to the 41st Millenium. We live in an (overall) optimistic time where science has solved many of our problems. We have free speech and are encouraged to question the world around us. It is quite reasonable to assume that we will one day live in a world where we have eradicated hunger and illness. In short 'nothing is sacred'. This isn't the setting of 40k, in fact, it's much the opposite. It's a paranoid, fearful place where innovation amounts to heresy. Daemons not only exist but actively seek to consume and destroy mankind itself, faith is the only defence against damnation. The Emperor was, is and always will be mankind's only hope. It is by his force of will alone that the Imperium endures at all. And his most important gift to mankind? The Space Marines, the Angels of death themselves.
So yes, It is, of course, possible for Guilliman to walk into this setting and wave his magic wand of science around and solve everything but is it in keeping with the 40K that we knew? This is essentially where the case against the Primaris Marines comes from and is the source of most people grievances.
Edited by Master Commander Ajax, 17 May 2017 - 04:28 PM.