There is time and there is time. Luffy vs someone like Lucci should be an easy fight for Luffy, perfect to showcase how far Luffy has grown. But you need stakes ALL THE TIME until the end of the series to keep the tension alive. After Kaido, Luffy seemed invencible to me... if the Admirals are an easy W for him the tension is gone. BB doesn't seen that treathning, Akainu and the rest of the admirals too... Shanks and Dragon are allies... Buggy is a clown and... yeah who else is left? IMU? I supose.
I'm not saying Kizaru should win here (that's bad writting) all I'm saying is that Kizaru should give Luffy a hard time here just like Doffy and Kata did before.
The fight shouldn't have to be that long but you get the message.