Luke did not win his duel by fighting Vader, he won by choosing to eventually not fight Vader, even though the rebellion depended on him assassinating Vader and the emperor. Had Luke struck down either Vader or the Emperor he would have drifted to the dark side and eventually becoming a sith himself.  Sith are active Jedi are passive, Luke won by doing nothing.  It's not the same as in DragonBall where Goku beats freezer because he's mad strong and strong mad, there is like a whole philosophy ingrained into star wars.  That said an unfit obesse Luke would never posses the preserverance and conviction in the Jedi ways required to just give up like that