Except if true something that important to the plot had to come from the story treatment. Written by George "plot convenience" Lucas, not JJ.
I think people are too quick to dismiss this as nonsense because it doesn't sound promising enough. While forgetting just how far GL's fingers are still stuck into this particular pie.
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Also, I honestly don't think it's terrible as a springboard for the plot, provided it is his mechanical hand. If they are making Luke the movie's McGuffin, they also have to justify why the young, unknown leads -instead of his friends- are out questing for him in the first place. And this way the Jedi among them will get a lightsaber at the same time. While a reason is given for why Luke even needs saving. I see no reason it can't be handled well.
I discuss to learn, not to win. Then again, learning enough tends to translate to victory in the end anyway.