But wasn't this problem already there by way of TFA? Isn't that the film that established that Luke left his sister to deal with the fallout from Ben's turn into Kylo?
What I'm trying to explain (and what I think you both already understand) is that Luke's character was damaged by having him leave in the first place. Going off to find an ancient Jedi Temple meant that Leia (and her Resistance) would be left on their own to take on Snoke, Kylo, and the FO. Luke was gone for years!!
I think we actually agree that Luke should never have taken himself off the board while evil ran amuk. But if that's what JJ set up, my whole point has been that if you can't undo the fact that Luke was gone while planets were being destroyed and Han was being murdered, the best you can do is provide some selfless motive for what he had already done. Something that makes rational sense.
If Luke came out in TLJ to help the Resistance (which he ended up doing . . . I know, I know), how would that erase the fact that he allowed the FO to wreak havoc? Why is TLJ being blamed for that?
I don't disagree with you. Luke not being there is all kinds of wrong and the blame in the first instance falls to TFA.
But, whilst TFA set up some ideas for Luke's whereabouts and disappearance, it was left to TLJ to cement that storyline. Luke was missing at these crucial moments, so as an audience we are left to speculate and wonder why? We can only guess that there must be some super important and logical reason for this... And here's where we differ because you accept that the answer provided by TLJ is the only logical one and I do not.
Upon leaving my viewing of TFA, I felt like the story could have gone a variety of different ways and had no idea where they were going to take it next. Here's some quick thoughts which are by no means amazing ideas but, to me, are still serviceable as follow ups to TFA (and in my opinion less likely to bring about such a negative reaction as TLJ):
(1) Perhaps Luke had gone searching for an ancient macguffin to defeat Snoke (a Sith holocron or Jedi text etc.).
(2) Maybe his spirit had been broken as a result of a terrible tragedy that befell him. I would imagine that any such tragedy would have to be one so utterly depressing as to render the most hopeful character from the OT now hopeless. For instance, I'm thinking something in the order of Luke's wife and/or child dieing, whether accidental or murdered, when the Knights of Ren burned down the school. Now Luke feels that there is nothing left to live for. His family represented the peace and love he had achieved in ROTJ, now all gone. Worst of all he had trained all the perpetrators, he made them into weapons for Snoke not Jedi. He sees that now and he blames himself. Luke has put his neck on the line countless times for the galaxy for the greater good but where was everyone else in his time of need.
(3) Maybe Luke no longer trusts himself and his own judgement. Snoke has proven that his reach and influence over both the force and people's will is incredible. Perhaps Ben wasnt his only victim, what if Luke too had been influenced by him?! Luke could have become a powerful pawn for Snoke without him ever being aware. Hence Luke cuts himself off from the force and goes in search of ... [go back to option 1].
TLJ sort of does something somewhere in the middle but never goes far enough to convince me. It also frames the trigger for his absence in a manner which I instantly found to be out of character (contemplates murdering his nephew) and follows that up another act which feels, to me, to be un-Luke-like (running away to die alone to end the Jedi to bring about balance).
Ok ok ok, I need to interject here and stress that this balance BS has been horribly retconned into the series, dark rising to meet the light ... That's not how the force works (well at least up until this point). But let's for the sake of argument pretend that is how it works now... Well ok, it still just doesn't work for me as a complete package explaining Luke's absence.
It feels like Rian is forcing the character to fit into a role in his story in the same way that someone might try to push together to north points of magnet. And like those magnets, the more I consider it, the stronger my reaction to it, I am repelled by it and I reject it. Just like I did in the cinema. Character comes first Rian, characters react to a story not the other way around.
After Luke slipped up with Kylo and the school was burned down, I believe (based on the Luke I have seen grow over the three OT movies) that Luke would do everything in his power to the right the wrong that he contributed towards (Ben's turn and the rise of Snoke). He would try to save his nephew (even if he thought there was little hope) but at the very least he would stay to support his sister and best friend Han unless the reason for leaving was sound. Once the job was done... Then he can take himself off to end the Jedi and test his balance BS theory. For heavensake don't run away and try out your hypothesis when the dark is at it's peak, your the freaking light. Go rise to meet it.
Ultimately both movies are a disservice to Luke but TLJ gets more grief because TFA only posed the question, it was TLJ that provided the definitive answer to that question. An answer which many have found unsatisfactory and unsavoury. It could have taken that answer many places, your mileage may vary but I think this is about the worst way it could have served Luke as a character.