The point of that climactic scene in Superman '78 is NOT that Jor-El was right, and that Clark fucks up by defying him. That's such an emotionally tone-deaf reading of the film that it beggars belief. The point is that this time, warnings be damned, Clark WILL NOT accept that he is helpless. He WILL NOT let what happened to Jonathan happen again. "All those powers, and I couldn't even save her" WILL NOT be the answer again. Not this time. Not this woman.
Superman is not a tale of accepting limitations and failure. It's a story of transcendence, of triumph, of being better than we are, better than we thought we could be. It's a story of moving Heaven and Earth for truth, for justice, for life and for love.
Is that "realistic"? Perhaps not. The fallacy is the assumption that realism is the purpose of narrative, or its highest mode of expression. Superman is not a documentary. He's a fable, a fantasy, a parable, an ideal. His world is better than ours, because he won't accept less, and he has the power and the will to make it so. He's the best aspirations of humanity, given fictional form and substance. Donner understood that, fundamentally and profoundly -- which is why his movie has resonated and defined the character of Superman, above all other portrayals in any medium, for more than 40 years.