How do you know that it cant happen? i have never completed a legion playthrough so i have not seen the ending slides.
"Civilization, unforgiving as it was, finally came to the Mojave Wasteland." That's the quote for one of the ending slides if Caesar is alive and you side with the Legion
Caesar himself said that the Legion is nothing but a collection of tribes under one banner. He doesn't want that. He wants an Empire.
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The Legion falls in the category of evil, but necessary. Just like a nation wanting great change needs to sometimes deal with hardships or guerrillas fighting for a very noble idea sometimes kills a bunch of their own countrymen to make things easier on them. Does Caesar ABSOLUTELY NEED to have slaves? Of course not.
But what is Caesar's Legion right now in the Mojave? A semi-nomadic mega tribe. Does Caesar's Legion have any infrastructure in the area? Besides the Fort, no, they don't. What does NCR have? A very nice supply line called the Long 15. Camps all throughout the west. And some backing of Vegas. Even then, they're stretched thin. Forlorn Hope is being screwed, McCarran is being screwed, etc.
Even the Legate, at the end, says the Legion will swell with the number of slaves they would gather from the place. The Legion's performance is largely dependent on slaves. They require no pay except food, which would be a constant either way. They have the slave collars so loss of equipment isn't common. They can transport supplies and do work far more efficiently than a paid worker.
Slave labor is cheap, crudely effective, and if we ignore morality, very beneficial. Why they use women? Well for one, they don't. They use all POWs that are too old to become Legionaries and young enough to do work (20-30 I believe). But the big reason is: Everyone works to what they're good at. Men are, generally, better at combat. However, even if women are slaves, they do have that meritocratic view of, "you do what you're best at."
That's why Legion blacksmiths, dog trainers, and the Frumentarri exist. And that's why women like Siri are still allowed to practice their medical arts. Why the Legion stopped her training? I couldn't tell ya. I doubt Caesar ordered that though.
Of course, if you have an optimistic view of House's ending, he's best. But I don't so the next best thing is Legion. Once an Empire is created, then egalitarianism can be debated on.