It's not that I don't grasp the intended themes, it's that I don't think the themes work in this context. I know that Johnson was saying that Luke had gone fatalist and assumed the worst about the Skywalker and Jedi fates, but I disagree that that is a conclusion that Luke would reach after one failure, however tragic it may have been.
More importantly, however, I feel the decision to go that route ends up disrespecting the character and the storytelling predating the Sequels. By all indications, Luke tried to reestablish the Jedi Order once and then declared it a lost cause when things went sideways, which is genuinely out of character for him. Did he flee the battle of Hoth once his gunner died or snowspeeder got shot down? Did he stop training in the Force when he failed to lift his X-wing? Did he go full hermit and cut ties with everyone when he realized that Vader was targeting the people he cared about to lure him out? Did he put up his lightsaber for good after he lost his hand to Vader, much less learned of their connection? Did he assume that it was pointless to defy Palpatine when he revealed that the attack on the Death Star was giddily skipping right into his trap? Did he accept the Emperor's logic that he was destined for the Dark Side when his rage got the better of him (several times) in the Emperor's Throne room and he nearly killed his father? No. In each case he accepts what happened and moves on rather than assumed that it had fatalistic implications.
Thing is, not all failures are equal, and by that paragraph, you're putting them all on the same level. Luke being shot down, or failing to lift his X-wing (which he does give up on, technically, Yoda's the one who raises it) aren't on the same level as, say, Luke failing to defeat Vader in Cloud City. And there's a key difference between those failures, and Luke's failure with his own nephew. Not only does he fail to save Ben, Ben's the one who he insisted be trained. He makes the decision to restart the Jedi, and yet, history repeats itself. The Skywalker bloodline seems cursed, a new Empire is forming, he's lost his nephew, etc.
And I get that Johnson was trying to tell a story about Luke rediscovering his hope for the Jedi, but Luke's entire narrative role in the OT was to be that very hope. The very title "Return of the Jedi" refers to him and acts as a final bow for the trilogy. The Empire is falling and the Jedi are back. To then just skip over the efforts to reestablish the Jedi Order and go "yeah, Luke tried, failed, and gave up on that years ago, so he's the last Jedi again" is a narrative slap in the face that tries to reset the status quo to how it was before the final bow rather than accepting and elaborating on the way that the status quo was changed by it.
I'm sympathetic to that argument. One can reasonably question the leap from Ep. 6 (the Jedi have returned) to Ep. 8 (guess we're back to only having one Jedi in the galaxy). However, if we're talking about redundancies and resetting the status quo, that's more on TFA than Last Jedi. TFA is the one that gives us the not!Empire, who destroys the Republic (a Republic that we've never seen), so we can have the Empire (sorry, First Order) fight the Rebels (sorry, Resistance) again. Last Jedi actually does something with the conciet that the galaxy seems to be in a never-ending cycle of light and dark, whereas TFA gives us a single line in reference to this.
If TFA wasn't so derivative, I'd be more sympathetic to the notion of Last Jedi 'betraying' the end of Ep. 6, but TFA, I'd argue, already does that. Rise sure as hell does that, bringing Palpy back with nary an explanation. Last Jedi at least brings some thematic weight to it, whereas Palpy in Ep. 9 not only betrays Ep. 6, I'd argue it also betrays him in Ep. 3 as well (oh so NOW lightning harms you!)
As a final point on this, it's not an expectation of infallibility. It's a matter of inconsistency and working with the source material. Imagine for a minute that I got the reins to make a follow-up for Avatar the Last Airbender (for simplicity, assume that this happened instead of Legend of Korra). Think about how that story ended. Now imagine that when my series picks up, decades have passed, the "Phoenix King Reborn" is picking up steam and on the cusp of reigniting the old war...and Katara tells our young protagonist that she and Aang broke up years ago after the death of their firstborn to the Phoenix King's forces, and that he had been in hiding for nearly as long. Our protagonist ultimately finds him at the Central Air Temple...having given up on both reestablishing the Air Nomads and believes it would be better if the Avatar cycle ended, as the Phoenix King proved that he'd failed to end the war.
Is that a potentially interesting angle? Yes. Does that respect the established character of Aang or the orignal Last Airbender source material? Hell no, and the story I'm brushing over is certainly not the material that belongs in a time skip.
Well, thing is, I don't really have a problem with that. I'm a bit wary of it being the Fire Nation again (in this scenario, I'd choose the Earth Kingdom - have the world be stuck in a state of war), but aside from that, no, I don't really have an issue with it. There's of course a leap from Aang at the end of Book 3 to how he is there, but a lot can change in three decades. There'd be similar outrage, but I could see that story working really well, and maybe have the setting's tenents question (should the Avatar Cycle end? Is having an Avatar a good thing? Is relying on the Avatar maybe hindering everyday people's abilities to get shit done?) I mean, there's 10,000 years of Avatars, I'd love to see that kind of story with a prior one.
By its very nature, the events covered in a time skip are supposed to be those that you can reasonably fill in the gaps for. The idea can basically be summed up as the timeskip being an "autopilot period". While he presumably had more adventures in the intervening years, it doesn't take much imagination to trace how Justice League's Batman becomes Batman Beyond's Bruce Wayne. That Bruce feels like a natural continuation of JL's Batman, with the main oddity being why he's so alone, but even that feels reasonable. Similarly, General Organa feels like a natural evolution of Princess Leia. In Jumanji, when you see what happened to Alan and Sarah after the prologue, Alan feels like the natural result of a kid who spent decades living alone in a killer jungle, while Sarah feels like the natural result of a kid who ran out in hysterics (claiming to have been chased by non-native bats) that claimed to have seen a game board eat the boy who disappeared. In X-Men (2000 film), the prologue gives us most of the information we need to infer how Eric (a holocaust survivor) would become Magneto (who is once again seeing his people - mutants this time - being systematically scapegoated and targeted in an uncomfortably familiar way).
Well, again, not all those character trajectories are identical. Bruce gets a line from Justice League to Batman Beyond, while the arcs of the Jumanji characters happen entirely in one film, while in X-Men 2000, this is the first time we see Magneto at all - any past trajectory occurs off-screen.
I mean, if we're talking about Bruce Wayne, and trajectories, and giving up hope, I can point to BvS Bruce, who's fought crime in Gotham for decades, and by this point, has given into despair, has allowed himself to become more vicious, etc. I think Last Jedi does it better, but one can easily draw parallels in concept.
As a general rule, the trajectory a character followed during a time skip should be easy to trace considering Points A (how they acted pre-time skip) and B (where they were at the start of the time skip), and Point C (where they pick up again). If you have to introduce significant developments during the time skip to justify a swerve in trajectory between B and C, that's a pretty good indication that that shouldn't be an event that occurs during the time skip, lest it feels like you're just unwilling to work with the character you were given.
Okay, fair enough, but I disagree that we're missing much. Not every part of a character's life has to be depicted, and Last Jedi does show us via flashback what happened. It actually shows us three times, really. You could, perhaps, have a hypothetical movie that depicts everything Last Jedi deals with in flashback, but I disagree that it's required. How Luke fell is documented in the film itself. And while I agree that not all time skips are equal, the Star Wars movies have always had time skips, often between movies (there's a ten year gap between Ep. 1 and 2 for instance, do we need to see Anakin's training there? Do we need to see the three year gap between Hope and Empire, explaining how Luke's abilities improve to the point that he can use telekinesis?)