Shanks' character resolves around the fact that he had the potential to be the new Roger when he was at max of his possibilities(2arms), but he decided to give up one of his arm, limiting his possibilities, because he found someone who could do better than him in Luffy, which alternatively also probably pissed off Mihawk at the moment, who since then was wondering who that Luffy could be(until he met him in baratie and ever since had been interested to watch if he really had that potential his rival gave his own up to and was betting on like he tried to see in marineford), ever since wandering all over the world out of sheer boredom and disappointment while looking for another rival to entertain him like Shanks ever could have because he losing a rival in Shanks also lost made him lose his life motivation until he found Zoro and was curious about his potential, waiting to fight him at his full potential by placing all his hopes on him to surpass him.
Tl;dr Shanks had the potential to be higher than this, but he restricted this potential by betting it on the new age(like he said to Whitebeard) and as a result is weaker than he could have been. It's potential wise that Shanks could have been the strongest, but because he wasted this potential, people like Kaido or Mihawk are stronger than him