I like to anticipate things from a writer's perspective. Would kishimoto allow his main character to alter himself to have a sharingan? Would he allow his main jounin to alter himself (in a way that the audience hadn't become comfortable with) to support this plot change?
It would be too drastic of a shift in the natural characterstics of major characters to give naruto a sharingan. It would be hideous, and it would have a strong response both positively and negatively.