First to get it out of the way. I didn’t particularly like TLJ but not because of Luke. I didnt like it because the Canto / hacker plan was very Independence Day and because both the chase and end battle seemed like a colossal tactical failure on the part of the FO and very Deus Ex Machina for the Resistance.
But anyway, moving on - about Luke’s despondency. Contrary to what I assume is popular opinion, I actually felt that it was fitting and inevitable. Even if they actually skipped portraying Luke’s transition between ROTJ and the new movies.
I see it much like Kreia saw it in KOTOR 2. The Jedi build, they establish their codes, then the Sith show up, because the Force always balances and as many turn to the light, there will be as many that turn to the dark. They always end up fighting, and the rest of the galaxy burns. The cycle repeats during the Mandalorian Wars, KOTOR 1/2, and all 3 trilogies. Masters and Apprentices and Masters and Apprentices.
What do the Jedi and what does the Force really contribute to anyway? It might be better to let them all die off. And yet the Force won’t let itself be extinguished. Which is really the oppressive thing about it. It really wants to be fed more lives.
Its like the old super villains exist because of super heroes and vice versa trope.
That, I think, is what Luke is struggling with.