That's true. In the end, my primary source is just my belief that Luke couldn't give in to the Dark Side, because he's the "good guy" (and because it would make Dark Empire more believable, which is unacceptable to me). Let's move on.
Why do you find Dark Empire unacceptable? I personally thought it was a great sotry,and remember, it was written before any of the chilidsh "chosen one" crap was made up. I personally think Luke is more Jedi than Anakin will ever be, even though hes only the "catalyst" that allowed Anakin to complete the prophecy.
Also, I think Dark Empire was a great story, GL himself said it was the closest thing to a sequel trilogy there is. What do you have against it?