You just want the gorosei to be major villains, but they’re just geezer subordinates. People had the same thought of Jack back in the day after he was the first billion berry commander at the time.
Saturns not a major villain. Kizarus role in Luffys story is much more substantial
How can you even compare Saturn's role to Jack's. I can also just say that Kizaru "is just the first of Akainu's subordinates to go", which would make him as important as Blueno.
Kizaru is more involved in the current arc, but the Gorosei are more important for the overall plot of the world and it's not even a contest: we just don't have much info on them yet and that's intentional by Oda.
As I said, Saturn may not be the final fight of the arc if the robot holds him down, but he's not randomly dying at the hands of the robot.