Luffy is still fighting so he technically didnt lose even once... Am I doing it right?
Kaido's legend is that he lost 8 times in his life, Oda is going after Luffy losing 8 times to Kaido alone.
Luffy came back because he is one of many stalling devices for Kaido until Zoro comes back to do what's necessary.
He can lose 100 times and return, doesnt change the fact that it isnt his place to end Kaido. That's Zoro's job.
Kaido is Zoro's prey, a device that makes Zoro's name reach the heaven. Thanks to Kaido, Zoro's name will shake the world.
It's not obvious that all samurai failed to slay a dragon and that Zoro will do it?