Nah, it's not optimism. I've been in this position of thinking I could change a person and "save" them. That was my arrogance. There is nothing optimistic about it. I know that now, looking back on that experience with my ever progressing age. Luke tried to change Vader in RotJ. He said as much...there was good still in him and Luke could turn him. There was also arrogance on the part of the Emperor thinking he and Vader together could turn Luke (or is that optimism on his part, too?).What you call arrogance, I call optimism.
I like stories with characters that have a hopeful, optimistic point of view. It's a big part of who Luke is. If I wanted a constant barrage of pessimism I would just watch the news.
Luke represented the best of what a person could be, and it's the breaking down of that character trait that hurt the most. The ridiculous throwaway line that went like "You don't need Luke Skywalker. What did you think was going to happen here? Did you think I was going to walk out with a laser sword and take down the whole First Order?"
Luke was never asked to take on the entire empire on his own previously. Every major victory against the empire was a team effort. He knows this. For him to suggest that his help was sought so that everyone could relax while he went to win against the first order is insanely bad out of character writing. The line was almost like a message to the audience - 'how foolish of you to believe in heroes'.
No hero is infallible. Trying to kill Ben Solo was what made him run away. That scared the ever-loving crap out of him, as it should have.
Luke was never asked previously to take on the Empire on his own, but he tried. Instead of rendezvousing with the fleet in TESB and then mounting a rescue of Han, Leia, Chewie and the droids, he went at it alone. Against Vader, against the Empire's forces on Bespin and without being fully trained. That was arrogance, that he could take on that group alone. Rushing into a situation without exploring all the possibilities and thinking you can solve everything isn't optimism. That's foolhardiness.