Your problem is you're trying too damn hard to equate JK Rowling's world with the real worldIts not about his acting chops. I could tell within 3 minutes of looking through his IMDB that he’s a top rate talent. Its about the CHARACTER and the fact that unfortunately when you race swap a character you have to take into account what that CHARACTER entails.
Snape is, for all intents and purposes, a Nazi. A repentant Nazi, but a Nazi nonetheless. A Nazi who only reformed because the unrequited love of his life, a WHITE WOMAN, dies violently at the hands of the wizarding equivalent of Hitler. This same character is abused by three white boys, hung from a tree, with his pants stripped down to his ankles. And THEN in a fit of anger, shame, and self loathing he calls the unrequited love of his life the wizarding equivalent of the N word.
As a white character, there is proper context to all of this. As a BLACK character it comez across stereotypical, c00nish, and ill-advised.
He wasn't hung from a tree with rope. He was levitated upside down, a typical prank wizard kids did to each other
James and Snape's rivalry wasn't based on skin color
Snape was Slytherin, James was Gryffindor
Snape was into the dark arts, James hated the dark arts
James was popular and talented in Sports, Snape was talented academically but a outcast until he befriends future death eaters
Snape becomes a death eater, James joins the Order
This doesn't change with a black Snape
But whatever, people just believe that these are just white only roles