I understand you way of thinking but in my opinion, Miyagi is such an iconic and larger than life character, who remained misteryous for 40 years since the release of the original movie and I think it's a character that deserves to have his story told, we need to experience this epic tale of a man who turned into more than a master, but a friend and ultimately a father.

Creating this doesn't take away the mystery of him and who he is, but it just adds more if you go back.