My guy, you realize you are setting up expectations for yourself that don't exist right? Kizaru going down now does nothing against the Navy not having their Marine force for the big war. Kizaru going down now just services the plot for Luffy. There's no reason for him to come here now to give Luffy his blessing about Vegapunk. Sentoumaru is already doing that in this case, so he'd literally be wasting his time traveling half a world to do the same thing. And escaping just isn't an option at this point, considering Luffy has mentioned not running away anymore from Admirals. He was forced to last time and is now a Yonko....
Kizaru doesn't need get demoted if loses to a Yonko. Moria wasn't removed from Shichibukai status for example when we got Marineford. We also technically don't know Aokiji's play here, so if he HAD to be replaced, there you go.