Well the I thought the what if qui gon thread was stupid for the same reason I mentioned above. There is no answer to what if questions cause noone knows the answer
Well Luke was there and Vader didnt have to do it but he did. He was the only one who could do it. And by doing it he forefilled the prophecy of the chosen one
they only debated the what if for a page the rest is about fighting styles of double bladed and single bladed swords.
To answer your question about the what if, any what if questions about the past is useless cause no one is right and no one is wrong which means it is pointless to debate it cause it has no ending.