In-universe, the story of Luke redeeming his father (who was basically space Satan) was super fucking impressive, and spread far and wide. They talk about this in TLJ.
Also, from a story point of view, the smaller more emotional climax of him redeeming space Hitler is far more cathartic and interesting as an audience (at least to me).
If Luke replaced Wedge in the final battle and blew up the core of the Death Star, he may have made a more apparent strategic difference, but his arc from the whiny farmboy in the first movie would be incomplete. He wouldn’t have used any of his new skills or anything he learned as a character if he just blew up the place.
If I was gonna change anything about the Luke stuff in the movie, I would have made the Emperor actually tempt him a little, because nothing he offers actually truly budges Luke.
But why would Luke’s redeeming Vader be the stuff of legends? It’s legendary to us - to fans - but why would it apply in-universe? We seem to forget that Vader was actually evil, possibly because we (again as fans) find him so cool no matter who he’s strangling. Every member of the rebellion would’ve lost friends and family to the Imperial juggernaut under Lord Vader and his Emperor. Do you honestly think folks would be praising Luke’s giving this man a bedside conversion? No, most people would be like “why didn’t you kill that monster when you had the chance?”.
The only ‘legend’ that could possibly arise from the RoTJ battle would be the notion that Luke killed the Emperor before he could escape the exploding Death Star. That’s the only story that could logically resonate with the populace. Rey’s version as told in TLJ is ridiculous. I can only assume that Leia told Rey about Anakin’s redemption before she left the Resistance Base, because again it would be absurd (and suicidal I’d expect) for Luke to have divulged the truth about his father to the wider populace.
Prior to TLJ, when RJ and Mark Hamill were hinting at revelations that would rock the franchise, I thought this was what they were referring to - that Luke would admit he hadn’t killed the Emperor as per the legend and that his entire fracas on the Death Star had been an insular pursuit. This would at least be something worth feeling guilty about.