It would be poor writing if Admiral Kizaru went down in this arc before we've even gotten to the final battle. There is already a shortage of Admirals who are antagonists given Fujitora is already a Luffy fanboy. Having Kizaru go down, reduces his hype which I doubt Oda would do. And considering Luffy has already defeated someone stronger than Kizaru i.e Kaido, he doesn't need to prove himself.