Go out and murder his nephew properly? Someone doesn't get Luke. Sigh, indeed.
Considering I was arguing that it's plausible that he wouldn't, all it means is that you didn't understand what I said.
I have a better analogy. It would be more like if in IW, Thanos made himself known and started destroying the world looking for the Infinity Stones, and while everyone is gearing up to defend the galaxy, Tony Stark is no where to be seen. When they do find him, he's pouting like a baby because his actions may have helped get his best friend, Rhodes, almost dead and definitely paralyzed. In the end, he sends one of his suits to some weak ass battle that accomplishes little, but then he dies by his own hand.
Not only would people not accept that and hate on the film, but it betrays his character. He is an optimist, just like Luke. Instead, he made his friend an exoskeleton that allows him to walk, again. The same goes for Luke. He would not go to some island to pout like a baby. He'd do something about his screw up. Even if it meant he had to die in the process of trying to save his nephew and the galaxy.
Do you know anything about Iron Man's history? The guy was driven into alcoholism under the stress of being Iron Man and pressure being put on his business. He literally was a homeless drunk on the street for a time, much worse off than Luke ever became.
In the MCU, Tony Stark breaking down under the stress and quitting is not only possible, but his PTSD from the Battle of New York has left him very erratic, switching between creating murder bots, putting a leash on himself and his fellow heroes, and, yes, even outright quitting as Iron Man for a time (something they never bothered to address).
Luke loves his sister and his best friends, and people in general. He risked his life to save theirs when both Yoda and Obi Wan said he may need to sacrifice them so he may become strong enough. He's not just going to cry in the corner when Snoke and the Order are going to kill billions, if not trillions, of people if they try to resist. Including the people he loves and would give his life for.
That was 30+ years ago. Isn't it just the least bit possible that Luke had lost faith in himself? That the certainty of youth had burned itself out and the magnitude of his failure left him broken? When Rey comes to him, he flat out says that he's got nothing to contribute. That he can't just wave his laser sword and turn the tide of the war. Except...
Rian Johnson may have a boner for nihilism and subversion for subversion sake, but that doesn't make him deep. And it sure as hell doesn't make him right
And here's where you miss the most important part. Rian Johnson wasn't being nihilistic or subversive at all. Throughout the story, yes Luke doubts his value as a legend. This is true. He did something great once, but he's realized that he's just a man. But you missed the moral of the story. Luke's greatest legacy is his value as a legend, and that remains untouched by the failings of the man. The children sitting around late at night telling stories of Luke's bravery are his legacy, the passing down of the Jedi way. He's a beacon of hope moving forward that generations will follow. He could wave his laser sword and make a difference, even if he couldn't singlehandedly crush the First Order by himself.
Johnson, far from subverting everything Luke represents, tells a tale challenging the notion of a legend but ultimately verifying its value and durability. Luke Skywalker is an idea that cannot be destroyed. Not even by the man himself.
Luke's story is easily one of the highlights of The Last Jedi. It's unfortunately that anyone would cross their arms at the very idea of a less than perfect Luke and dismiss it just based on that.