Regardless, the heavy lifting of the characters success was based on Supermans image - white, tall, male, deep voice and athletically fit. Thats a fact. To deviate significantly from any of this visual template is disrespecting half the character. Giving it to a black actor or a chubby actor or a ugly actor or a female actor (yes Supergirl is just a female tokinised superman) is cheap and an unearned accolade worthy of holding the Superman legacy. In the same way casting a white man for Blade is. Writers of Blade fought hard for decades to push the character in the mainstream. After finally being accepted and loved for who he is, aesthetic included , you think people would be okay if he was cast with a white actor? Would look like white saviour complex, swooping in to steal the kill from a black comic character whose worked hard to get to a high status. Same is true for superman or any other fictional character