Eh, I’m with Dom on this one. We knew nothing of Vader’s turn in TESB. Why the double standards?
For me it is, because Ben’s turn is intimately connected to Luke’s fall from grace. When we first saw Vader, and his mentor Obi-Wan, the story was about Luke’s journey, and we didn’t need to know the backstory in detail. They were afterall new characters. However, since this is the 8th episode an ongoing series the story needs to naturally progress. To me the ST is like having viewed the PT, and then have Anakin Skywalker be good at the start of ANH with a short flashback in TESB telling us how he went from Vader back to Anakin. That’s not natural story progression. If you look at what happened between the PT and the OT, the answer is very little. The Empire increased their hold on the galaxy, the last Jedi were hunted down, and a Rebel Alliance was formed, but the characters were pretty much in the same place, often literally. Not so for the ST. It’s like there’s a trilogy missing, leading many to feel Luke’s character change has not been properly developed. Since the developments around Luke’s character are strongly connected with young Ben’s character development, I needed more development for both of these characters. So, in my view it’s not a double standard.