Saturn's role is to give the readers an idea of how strong the Gorosei and Imu are.
That magic circle screams "a final villain has just arrived".
Saturn isn't getting defeated here or it will massively dehype the Gorosei and Imu.
With Saturn being present in the battlefield, the protagonists have no way of winning the battle.
The Pacifistas are already on Saturn's side.
Luffy is tired.
Zoro is occupied.
Jimbo is far.
Sanji isn't defeating Saturn.
Things will only get worse if the Seraphs gets freed.
The SHPs have no other choice here but to flee.
Saturn will get stalled by the huge ass robot while the SHP and co. escapes.
My hopium is Dragon and some other Revos that can fly will come and help the SHPs and VP escape.

