I agree. There could be an argument for any number of things keeping Luke on that island, but fear is on a very short list of wrong answers.
My own interpretation, based on Luke’s entire life, is that he is there because of pain.
Throughout the OT, Luke has faced fear in many forms. He didn’t let his fear of the Death Star stop him from going back and destroying it, nor did he let his fear of Vader stop him from facing this villain in Cloud City. He even accepted his fear of death when he went to the Emperor fully accepting his fate.
No, what Luke can’t bear is seeing his family dying, his friends in pain. This motivates him throughout the OT, so if Luke were to see his friends destroyed in Ben’s future, he would do almost anything to stop it. In this it makes sense for him to contemplate destroying his nephew, but it also makes sense for him to realize the paradox of his action. He cannot kill his nephew, but in doing so he dooms his friends.
He flees into exile not because he fears making this choice, or if he does the fear is fleeting, but rather because he will not choose, cannot choose. This is where I believe TLJ got Luke wrong as well. There is no shame in shrinking from an impossible choice. Ben cannot be turned, at least by Luke, and so there is no stopping the dark future that Luke sees. Any guilt he feels is misplaced, because Ben’s heart is already gone.
“I can’t get the vision out of my head…they’re my friends, I’ve got to help them!”
Except…this curse of vision will never end. Luke must cut himself off from the Force, it is the only way to stop the pain.
In the end, he is at peace because he accepts the paradox. Kylo is the relative he cannot redeem, Han and Leia are the friends and family he cannot save. But this is okay because he accepts what he cannot do and embraces what he can, though it lead to certain death.