What part of being the strongest don't you understand?
All characters in the verses are being defeated if they went through the same situations as the kaidou in Onigashima, let's not pretend that the ODA didn't have to wear out the kaido for his defeat to occur and be acceptable.
Or do you think another character, alive or dead, is overcoming everything kaidou has been through and beating luffy at the end of it all?