You know, I think it comes down to a flawed view on the person's humanity -if you want to call it that- that makes both Obi Wan and Yoda's assessment wrong.

You might be onto something with Luke, after all, it was his appeal to his father that brought Ani back to the light.

No, I think you are on the right train of thoughts. I do feel that the Jedi would have benefited of studying themselves and the dark side more.