Why? Vader was never really the villain to begin with, In episode 4 yes, Vader was definitely the guy they were fighting against, but in V and VI it's unequivocally Emperor Palpatine who is the main villain. In I, II, and III Vader isn't the villain, even when Palpatine recruits him in III, you can see Vader is just a disillusioned boy who is just trying to save his family. When he kills Palpatine in Vi, He essentially reverts from his role as Vader, to being Anakin Skywalker again. Vader redeeming himself by killing Palpatine is a huge plot point in VI. He finally gives up on his servitude to Palpatine to save his son. Also, why would anyone try to stop Luke from saving Vader? Palpatine didn't want Luke harmed when he was brought aboard the new Deathstar, so why wold they try to kill him when he is trying to save the person who is basically their second in command?
Han shooting first or not doesn't change the plot of the first movie. It just makes him seem less ruthless than the movie originally conveyed. Changing that one detail doesn't completely change his entire arc in the rest of the series, just that one facet of his character type. That is completely different than making Vader a villain again, when his entire character arc in VI is based around changing because he loves his son and cant bear to see him die.
Making Vader the villain again would not only reverse the entire ending to episode six (since you know, he is dead at the end of VI), but at the same time reverse his entire character arc, since sacrificing himself for his son was basically the culmination of his character.
I actually do hope they pave right over the EU. 90% of the EU was just retreads of of episodes IV,V, and VI, and I hope they can do something that is actually interesting in episodes VII, VIII, and IX.