The marines will rebel only when they learn that Nika is back and defeated Saturn, the World Government's strongest force (people don't know about Imu).
They'll realize that the World Government is not invincible and Saturn's defeat will start a revolution and trigger the final war.
Why do you think that the Egghead incident is going to shock the entire world?
What you say is the direct consequence of Saturn's defeat.
The biggest reason I doubt Saturn loses here is it goes against how Oda introduces strong characters and groups of strong characters.
Luffy meets a Shichibukai, loses twice before winning. Luffy meets an Admiral in Long Ring Long Land, loses horribly, fights an Admiral in Sabaody, loses horribly, gets roughed up by Admirals all through Marineford.
Oda wouldn't wait this long to introduce the Gorosei if they were weaker than the Admirals, and wouldn't make the Admirals serve them if they were weaker than the Admirals.
I think Saturn will take serious damage here, but only after fighting a ton of people, because it would ruin the hype/portrayal of the Gorosei. Like, Blackbeard has no hype for me if Luffy wins here.
Best I can see if Luffy knocking Saturn out the same way he knocked out Kizaru, but I don't see him doing lasting damage, especially since the Gorosei seem to be inhuman in terms of their regeneration abilities (or at least Saturn is).
Saturn is a hypetool for the Gorosei, so, I expect him to high hard, get knocked out of the fight, but still be relatively fine afterwards.
I also just don't want the Marines to turn on the WG because they're losing, I want to see the Marines turn on the WG because they're fucking assholes.