Shanks is a genius; In this manga, it's rare to come across a true genius since most characters get strong through pure effort and constant training. Shanks is a prodigy and should have continued that way, but losing his arm should have been monumental news. Shanks was the only one capable of learning Kamusari, even though he was very young in the crew compared to the others. Gaban himself told him he was the 'child of destiny,' which means that when Shanks learned Kamusari, it seemed logical to everyone because he was destined for the world's peak in terms of strength.
It's such a shame he lost his arm. Even Whitebeard, who was amazed by the legendary duels between Mihawk and Shanks, was shocked by the loss of that arm. Mihawk had found peace being at the top of the world, but he enjoyed having someone as powerful as Shanks as a rival; the news must have hit him incredibly hard. The legendary duels between these two powerful rivals had come to an end