Between 1983 and 2004, the apparition of Shaw has not bothered anyone, everyone understood he obviously was Anakin and NOBODY never said, before 2004: "Hey, it’s not logical."
I was 9 in 1983, and I didn’t have to ask to my father “Hey, who is this ghost next to Obi Wan ?”. He was the ghost of Anakin, and it was obvious because I already saw Shaw’s face 5 minutes before, when Anakin died in Luke’s hands.

I do not understand how replacing Shaw with Christensen in 2004 could make more sense of something that made perfect sense without this change …