To me, what TLJ did with Luke was the logical conclusion of what was set up in TFA. In TFA, it was established that Luke went into seclusion after Kylo turned, abandoning the resistance and the way of the Jedi. The notion that Rey would meet Luke and suddenly all the reasons he went into seclusion would melt away was not even remotely hinted at in TFA. I didn't find his bitterness and cynicism unexpected in the least, based on what we learned in TFA. Furthermore, I thought he was actually very consistent with the Luke from the Original Trilogy. In the OT, he started as a whiny, brash, impatient kid who had many heroic moments but whose impatience repeatedly had significant consequences throughout the trilogy. It makes perfect sense that someone whose impatience has burned him throughout the course of his life would reach a breaking point where he gives up and becomes jaded, bitter, and inert - the other side of the whiny, brash and impatient coin. Luke's final act in TLJ was, in my opinion, incredibly fitting. With Yoda having finally gotten through to Luke, he ultimately finds balance - he doesn't let his bitterness continue to rule him but he also doesn't revert to rushing in recklessly. He transcends his failures and his final act of heroism is one of calculated self-sacrifice, sage wisdom, and focused power. None of his other heroic acts in the original trilogy achieved all of that.