Maybe I should have been more clear. I'm responding specifically to the idea that Saitama isn't allowed to hurt Superman because that would ruin Superman's "gimmick".
Saitama as a character is defined by the fact that every fight he gets in is comedically unsatisfying for him. All he wants is the rush of a good fight, but he has grown too strong for that, and now he has to find something that actually makes him a happier person instead of chasing some dream of power and excellence to give his life meaning and distract him from how unhappy he is. Having Saitama lose a fight completely and fundamentally changes the nature of his character, because now the idea that he still has some enemy he needs to grow stronger to overcome is back on the table.
Superman doesn't have 'has to win effortlessly every time, no exceptions' baked into his character on that same level. Because, again, Superman loses fights. Not most fights, but at least some fights. Its a thing that can happen for him, unlike Saitama.