You said it was Rian Johnson's idea because that's what YouTube indoctrinated you with. You're ignoring that the whole thing was George's idea, which those YouTubers also ignore while screaming for Disney to bring Lucas back.The prequel trilogy proves what Lucas wants isn't always good either.
The movie needed a lot more to sell you on a hermit and cowardly Luke Skywalker than a few minutes of flashbacks. I'm sure you remember even Mark Hamill questioning how that optimistic Luke turned into a hermit in interviews.