I could definitely see the emotional aspect of it. But to me what made it perfect was the closure.
A rare case of closing the book on a story and setting up a sequel without being too hamfisted.
Regardless of whether you kill Shimura or spare him your fate is sealed: the Shogunate views you as a traitor and will now have to live your days out on the run or in exile. In essence, your actions lead you to morph into the Ghost of Tsushima since because of what you did, you must remain unseen. You yourself become the the very myth spoken of by the people of the island.
A rare case of closing the book on a story and setting up a sequel without being too hamfisted.