The gorosei are important to the world building, but they’re not important to luffys story. Especially not compared to Kizaru who Luffy has held a grudge against since Sabaody.
Saturn isn’t Luffys fight here. It’s Kizaru.
And that's not the point I'm arguing against. I suggested he logically must be (as both Kizaru's superior, probably the stronger foe, and the one in control of the Pacifistas), but I soon conceded that if the Robot holds Saturn and Luffy takes down Kizaru they can find a way to escape despite the Pacifistas and whatnot, so I'm not against Kizaru being the final fight here (which still wouldn't make him the major villain in my book, but it can be if you consider Katakuri WCI's major villain. I don't).
What I'm arguing is about Saturn just randomly dying to the robot. He's not important to Luffy's story
yet. Kizaru is just some dude he met once before and defeated his crew, but he wasn't even what marked Luffy before the timeskip: that was Marineford. It's not really that deep. After this arc, Saturn will also be some dude Luffy met once before, while still being more important for the big picture.
Saturn doesn't even need to go down to Luffy at any point: he can go down to Zoro, Sabo, Sanji, Smoker, Aokiji, I do not care, but I'm positive he's way too important to die here to an Ancient Robot (regardless of power levels), it simply doesn't fit the story telling.