I think there are multiple ways Johnson could have taken Luke. Multiple different explanations for his exile. But the biggest problem with the one he chose was that there was insufficient explanation of Luke's fall. Luke goes from refusing the dark side in his refusal to kill his mass murderer father to being willing to kill an innocent kid. That transition was poorly explained.
If they'd ditched the Canto Bight nonsense and spent time really exploring Luke's fall from grace and his motivations then I think it would have gone a long way towards calming some of the ire and made for a better movie. I think this was a failure of storytelling.
I think JJ was setting up Luke's exile to be one of protecting his friends. He knew Kylo wouldn't stop looking for him so he went and hid to keep others safe. Throw in a dash of guilt over Kylo's fall and you get Luke to where he was. Not a particularly interesting story and something of a retread (JJ seemed intent on retreading everything he could) but it's where JJ was going.
I could accept Luke falling to a bitter exile if it was just better fleshed out. His fall wasn't believable.
The transition isn't poorly explained if you take all sort of information.
1. Luke almost killed his father, before stopping and rejecting the dark side. He tapped into the dark side, chopped his father's hand off and was about to kill him,but then stopped.
2. What makes the Skywalker's special is their command of the dark and the light. Their touch with both, this was established in the Mortis arc.
3. A line from the ESB "Yes, run! Yes, a Jedi's strength flows from the Force. But beware of the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice." Luke started down the dark side path but wasn't to continue down it as he was able to turn it back but it's not like it totally would leave him. That's a silly thing.
Also the set up was there. Luke saw that Kylo was turned, and saw that Kylo would destroy everything he fought for and created, and as he said "instinct took hold, just for a second" it's hardly impossible.
There were other options that exist I will admit it. But it was the logical guess with everything else. People didn't want it. But the logical thing was to go into exile. Everyone assumed it was Yoda. But even more extreme. Or Ben. And it could have been. But it would have been a leap.
JJ may have intended it that way. But he provided no information on details. Other than a student destroyed his school and he ran away in exile. Like Leia had no information, which is stupid. But anyway, I digress. Luke could have gone a number of ways. But with all the information from all the mediums, it was a logical place to go.
That being said, I believe you're allowed to have your opinion, I just don't agree with it and I think that's fair. Everyone should be able to draw their own opinion.
Again, the clearly better option is Luke's Jedi Order is under attack by the end of the first act of the movie and he dies in it. That's the clearly superior method of telling his tale. He dies protecting Rey who goes on the run, then you can even throw in a time jump if you want. But even without it, it would give Rey an opportunity to seek out the resistance and han as they'd be the protection against the first order.
