Here's what going to happen.
Luffy will beat Kizaru.
But it will be a good enough fight that the future Zoro/Sanji admiral fights are still exciting.
Saturn will show up when the navy is on the backfoot/defeated, flex for a little bit, and then leave with the seraphim.
I'd argue that this is the most logical sequence of events but then again the BBP are here so who knows.