Also it affects the way people treat Luke. If it was common knowledge that Anakin had turned to the dark side and become a sith (and everyone would have found out especially amongst the rebels) then they would be very wary of Luke. However if Anakin is still remembered as a great hero of the old republic then they would be far more welcoming to Luke.

Yes.. and that's how it ALREADY IS.

He's not changing a "everyone should be wary of Luke but isn't" plot into "now under my rewrite, no one actually knows" - ALREADY no one knows, and he's being redundant.

How hard is that to grasp?