It was also the only way Lucas found to get himself out of a hole he had written himself into. He needed that as Luke's motivation to go apeshit on Vader.

Luke's under the gantry hiding, won't fight Vader and would rather die first, there is no motivation for him to tap into his anger and go over to the darkside.

I mean he is fighting his father, not his father's killer. Total change in motivation. If Vader really killed Luke's father and also Obi Wan all you would have is a Revenge tale.

Luke finally in the end throws his saber away realizing he is becoming his father. Looks at his own mechanical hand and vader's severed stump with wires coming out of it.

He proves to be of stronger character than either of his parents from those prequel films.