Before the prequels, Vader’s redemption worked for me because it was really all about Luke meeting and briefly getting to know his father. It was the completion of Luke’s arc that was setup in the first film, and wasn’t really about Anakin/Vader. The prequels completely changed the focus of the redemption to Anakin/Vader and broke it.

Actually, making Anakin the main character in the prequels is the main source of a lot of the problems with them. I get that it brings a nice symmetry and cohesiveness to say that the movies are about the Skywalker family, but the prequels should have been Obi-Wan’s story. The OT frames Vader’s downfall as being more about Obi-Wan’s own hubris and failures as a teacher.