The ST is beholden to the preestablished lore, and should be logically consistent.
So you’re saying the FO could rise to the way they are?
Given the vigilance you would expect from the Galaxy against the Empire returning to full power and building another massive planet killer, no, they likely couldn’t.
The Empire was reduced to a small fringe faction. The fact they had the massive amount of resources needed to produce a galaxy-seizing military and Starkiller Base, all without the Republic noticing, stretches the viewer’s belief.
Also, my examples are incompatible. How did the Empire come into existence when they clearly didn’t exist during the “before”? How can a new faction somehow overthrow a thousand generations old Jedi Knights and Republic?
Because that’s how time works. “Before” and “After” will always be different because the only constant in reality is change.
And when Star Wars first came out, we didn’t know the details of this history. But, since Star Wars wasn’t a sequel to anything, we didn’t need to. What we saw in ANH was the universe as it was being introduced to us. So it could be whatever sort of universe the story required.