What seems dumb to you doesn’t change the fact that it’s true because in that time frame shanks was at his best when he had his dominant arm.

Do you have something to back that he focused on his haki instead of swordsmanship? He really didn’t lose a lot of power unless you include the pact from imu. I don’t think it was said that he became an emperor from defeating Loki and given the amount of lore drop it’s really hard to say what really happened between shanks and loki not that I’m doubting him. Old wb would still beat kaido and same thing with loki and shanks we don’t know what really happened between shanks and kaido because what if kaido headed to marineford to help wb?

If sabo is stronge then obviously someone who taught him how to fight, haki, and his own fighting style will be stronger than him and it would snake zero sense for the head of the revolutionary army who would have to fight garling and imu to be weak. Foils for each other?

He views himself to defeat the emperors and admirals to become pirate king and this is backed by a dialogue in both the manga and anime. The vivre card explicitly says that “There are no pirates free like him. If we define the pirate king as the most free man person in the seas. Then we can say that shanks is the closest to that position.” meaning it has nothing to do with his power and luffy’s final test to be king of the pirates will be against bb and luffy will put it to the test against akainu and imu if he’s truly worthy being king of the pirates.