If you don’t think he would act that way or be discouraged then I think you have missed something about the entire saga. Try watching TESB and TLJ back to back. Then tell me that TLJ isn’t in character. Yes, in ROTJ, he is more confident, but confidence can be shattered easily. Luke has shown that to be the case. Insisting that he maintain the confidence that he has for most of ROTJ is unrealistic. The unsure Luke we see in ANH and TESB is just hiding under the surface in ROTJ and only needed a sufficiently horrible event to bring it out.
In my view you missed something in the saga, because you keep going back to TESB to argue Luke in TLJ is consistent with his portrayal in the OT. The problem is you view Luke as this flawed static character. Luke in ROTJ is just the same character as in TESB with a little more confidence. Take that confidence away, and add a few years and you have Luke from TLJ. However, you ignore much of Luke’s character development in the OT. The Luke that let himself fall to his death rather than accept Vader’s offer, is not the same character as the one who left Yoda and Obi-Wan on Dagobah, and that’s without considering the time he’s had to reflect on the consequences of his actions. Then there’s his development in ROTJ, where after being goaded by Palpatine, and Vader to release his anger, at that seminal moment he realizes he will follow in his father’s footsteps, if he doesn’t let go of his fear, and anger. So, he throws away his weapon, and declares himself a Jedi. These are scenes of critical character development, that you just seem to ignore. The Luke at the end of ROTJ is not the impetuous naive, youth we met in ANH, nor is he the impatient hothead from TESB, nor is he the overconfident Jedi wannabe at the start of ROTJ. He is a fullfledged Jedi, in tune with the Force, and able to avoid emotions affiliated with the dark side of the, such as fear, anger and hatred. That is his arc in the OT, his hero’s journey. You say:
“Try watching TESB and TLJ back to back. Then tell me that TLJ isn’t in character.”
What you’re really saying for the Luke of TLJ to be in character, he has to regress back to the character he was, when he first entered the cave on Dagobah. To me that’s one of the running themes in the ST: regression. Han regressed back to smuggler, Leia regressed back to the leader of a fledgeling rebellion, the galaxy regressed back to the Galactic Civil War, and Luke regressed back to his younger stupid self.
No, I’m not saying Luke is static. I’m saying people have traits and those traits (good or bad) don’t just go away. Luke in TESB and Luke in ROTJ are one and the same, but different traits dominate. Luke in TLJ has reverted back to some of those old traits in TESB and ANH. They are some of his character flaws. And what I am saying is quite different from what you are saying. I’m saying that due to the horrible event of the destruction of the new Jedi academy, Luke has retreated back and let his character flaws show. Quite the opposite of what you seemed to want him to be that he would be unaffected by such a horrible event and maintain this Buddha like state of enlightenment without being impacted. You are the one saying he should not change when we are presented with events that would change just about anyone.
And Luke is not a full fledged Jedi in ROTJ. Yoda states that when Luke goes to see him. He is not a Jedi until the end of the movie when he throws down his lightsaber. Not until he faces Vader and the Emperor and stands fast to Yoda and Ben’s teachings is he a Jedi.
And what is wrong with regression when the need arises. The story setup is that a powerful remnant of the Empire is now threatening the Republic. Leia would naturally take up that challenge. And if Leia and Han split up, why wouldn’t Han retreat to what he knows so well. As for Luke, it is only a horrific tragedy that has led him to regress (I wouldn’t call the others regressing but just circling back to things they are good at). You seem to have a problem that our world runs in cycles and that the ST is showing us that. A new generation faces similar challenges and how will they cope… how will the win. And the key moment that has led to TFA is the fall of Ben Solo. That even has triggered just about everything we are seeing in the ST. Mother, father, and uncle are deeply impacted by it and the Galaxy is as well because of how he fell. The premise is set in TFA and TLJ just follows through with it. TROS will complete the story and I think will make things make a lot more sense. They have not been telling a simple tale with the ST and I think some of the complexity will not be revealed until the end.
That Luke could not experience something to break him and bring out his character flaws really ignores the real impact of tragic events on people. Yes, Luke was the hero of the OT. But we are passed that now. Life does not end with “they all lived happily ever after” and it is not in the nature of the serials Lucas started from. Happiness and success only last until the next event. Why you expect Luke to remain exactly where he was at the end of ROTJ is beyond me. TLJ shows us a Luke who is wise and jaded. He knows the force, knows it’s lessons, knows the history of the Jedi and has cut himself off because he could not protect his nephew. He is not static by any means. He is a fleshed out character who is exactly what I would expect after such a tragedy. His youthful character flaws are still there, but the person they appear in is not the same. It creates a very rich and interesting character to watch on screen. And Mark played it perfectly. Much more interesting than what you have suggested. And, if I may say so, much like some of the characters in the early drafts of The Star Wars. So I find his the TLJ version of Luke to be absolutely perfect for what the character has been through and where he should be 30 years passed ROTJ.