I got your point... I just don't agree with it. ;)
Chaos is certainly powerful, but I wouldn't say it's the way forward. The powers of the Warp are too fickle, treacherous, and undependable to be relied upon.
Sure, they bestowed immortality to the Traitor Legionnaires--but only after a fashion. They remain immortal as long as they reside in a realm of insanity, which brings with it unwanted mutation. The World Eaters were driven to extremes of aggressive insanity, and it can be argued that the Emperor's Children were as well. Mortarion certainly didn't plan for his Death Guard to become plague carriers. And Ahriman, whatever his self-conciliatory notions, definitely didn't plan on turning 90% of his vulnerable brothers to mindless automata.
With the Emperor, the tyranny and oppression of the Imperium (traits that were by no means universal) had a point:
preventing the collapse and horrors of the Age of Strife (which, by the way, was due to Chaos) from occurring again. By contrast, the scale of suffering and insanity that Chaos espouses is aimed only at the pleasure of the so-called Ruinous Powers and the furtherment of their own might.
Chaos, ultimately, isn't a way forward for Humanity. It's a lateral progression toward corruption and/or insanity. The Imperium hasn't moved forward in ten millennia largely precisely because of a reverse of everything the Emperor espoused--they have mystified his person, espoused the religious and supernatural, and attached ritualization to virtually every facet of Humanity's existence. By contrast, have the Ruinous Powers allowed the Chaos Space Marines to make any great advances? Not really, excepting larger and more extravagant means of killing ever-greater numbers of (largely) innocent people.
Nothing about the Emperor indicated a "things as they have always been" agenda, though. Brutal, unrelenting, possessed of insufferable amounts of self righteousness, it still aimed at justifying those sins by freeing Humanity of the zero-sum game that Chaos had forced upon it (and other sentient species) for thirty thousand years.
EDIT (some spoilers below):
Furthermore, I don't believe any of the Traitor Primarchs were "cheated" or betrayed. I think the Emperor himself might
have erred by not trusting them with certain aspects of knowledge, but this hardly amounts to valid reasoning on their part. Rather, I believe it was other forces at play that corrupted them... and made it easier for the bitter pill of "betrayal" to be swallowed and be made plausible in their eyes.
Horus - Whatever issues he had with the Emperor, ultimately he needed to be wounded, poisoned, exposed to Chaotic rituals, and then fed visions of a future he
bring about in order to convince him of his father's "megalomania/envy".
Alpharius - As seen in "Legion", the Primarch of the XX Legion did not turn against the Emperor out of hate, resentment, etc.
Angron - Already dangerously insane and violent, he himself was led to Khorne's cult by Horus. "After Desh'ea" pretty much absolves the Emperor of the theories of betrayal wherein Angron wails about his warriors being left to die.
Fulgrim - Would need to be brainwashed by a daemonic weapon for an extensive period of time. Even then, when confronted with the magnitude of his actions, it wasn't enough. It's not really Fulgrim that leads the Emperor's Children, now, is it?
Magnus the Red - Never wanted to be an agent of Chaos. In fact for all his knowledge and sorcerous might, he proved to be grossly ignorant of the actual gods of Chaos. He only turned after Horus
changed the Emperor's orders to Leman Russ, which led to Prospero being decimated by the Space Wolves.
Mortarion - An intriguing case. We haven't actually seen the reasons for his fall yet. Hopefully, a "prequel" Death Guard novel will explain this further.
Night Haunter - Another dangerous sociopath, but his is a more interesting case. Here's a question: if the Haunter's chief complaint toward the Emperor was that he rebuked him from not stopping his excesses (which the Haunter claims were authorized), what does that say about the Haunter himself?
If the Haunter was even remotely
decent, being told to cease his actions should have come as a relief. Instead, he comes off as a wounded child, made angry because his father has finally told him he can't abuse and terrorize the other kids.
Perturabo - Probably the least sympathetic of the Primarchs, in my opinion. His "burden to bear" is basically having to do what had led to Guiliman and his Ultramarines being so hugely successful. The reason he couldn't do it might be due to his implied inability to trust anyone from as early as his childhood in Olympia (which might be a hint of even greater problems).
... and thus we come down to Lorgar.
It's not coincidental that Lorgar had the strongest case for "Primarch who turned because he was wronged by the Emperor"... he also happens to be the original
rebel and the instigator of the entire Heresy. Lorgar's chief complaint is that he is not allowed to exercise his faith.
I am actually willing to give this one to him. Of all the Emperor's failings, his greatest one seems to be his inability to relate to the Human Element on a micro level. By that, I mean that he is an excellent organizer, strategist, etc... but everything he does is on the macro level. He works on the scale of billions or trillions, not the individual. He orchestrates social and cultural changes that affect entire planets at a time, but cannot operate where an individual's emotions are concerned. He is aware
of such concepts, hence his use of "Father and Son" themes, but he is ultimately unable to execute them properly. He doesn't ever actually say "Son, I love you." He projects enough psychic force to overwhelm that person into feeling it. He doesn't ever sit down and explain why his son was wrong; he projects enough psychic force to make his anger/disappointment/resentment felt.
Maybe the Emperor is just so inhuman that his logic approaches that of a computer. To him, the statistical cost of Lorgar--a being of tremendous faith and religious zeal evern before being found--being told of the menace and danger of Chaos perhaps outweighed the statistical benefit of him receiving enough clarity to make an informed decision of loyalty toward his father. I don't know.
Without Lorgar, the Heresy doesn't happen. At best, he gets Angron (insane), Fulgrim (brainwashing via daemon weapon), Magnus (assuming Tzeentch throws his weight in), Night Haunter (maybe, it all depends) and Perturabo (depending on how much you buy the theory that Horus gave him a brainwashing weapon).
That's eight, instead of nine, Traitor Legions--at best--and that's assuming that Horus' charisma, which was cited as key for the Heresy to happen, wouldn't force any of them to change their mind.
OK, ramble over!