Diversity is not ruining movies.
Yes there are some cases in which movies producers choose to hire a black acor even though the character they are supposed to play is white. But for decades we've seen the opposite and no one complained.
When Auda Abu Tayi, an arab leader, was played by Alec Guiness, an english actor, no one complained.
When Liz Taylor was hired to play Egyptian Queen Cleopatra, no one complained.
But don't you dare have blacvk actors playing WW2 english soldiers, the alt-right won't accept it...