First, Clarks desire to save people was *never*not oncecalled into question.
Second, the film and its characters are not cynical at all. Its an honest moral dilemma: Is the world ready to learn about the existence of alien life? Could the worlds reaction to that revelation be so poor that the world ends up in worse shape even with all the positive things Superman brings to the world? If Clark were a real person, he would have to answer those questions for himself before choosing to reveal himself, and honestly its impossible to know the answer. It would be an impossibly difficult dilemma to resolve, and we see him struggle with that dilemma, and that adds gravity and import and a sense of earnestness to the film.
Costners scenes were not for nothing. His scenes were critical in establishing the difficulty of that dilemma faced by Clark. It wasnt unearned cynicism when Pa said, I dont know. Maybe. It was the voice of a Smallville farmer who truly didnt know if the world could handle someone like Clark existing. And honestly, if someone like Superman were to appear tomorrow, I think that person would fracture society; I think the world would be terrified and angry, and I think youd see mass riots, as the world started to question their own religious beliefs. It would trigger a momentous period of evolution and revolution of thought. For **** sake, it could lead to world war when major world powers frantically disagree on how Superman should be handled. I think its intellectually dishonest to call that outlook cynical. Its an inherently difficult dilemma that the filmmakers could choose to ignore, but that doesnt mean the dilemma wouldnt exist in real life. Is that what youd prefer? For that consideration to be ignored as if it didnt have to be taken into account?
I see confronting that dilemma as absolutely necessary. Anything less is cheating the story of a natural opportunity for depth and weight and verisimilitude.
In having Pa Kent establish that dilemma, the film allows young Clark to demonstrate his innate need to use his powers to help people, and how that need transcends such considerations (school bus scene, and the consequences of that scene); and it establishes it as an inherently difficult and brave and decent and inspiring act for Clark to go public as Superman when he finally does.