Sorry for the abstract mathematical phrasing, but this is the only way I can nail this point on the head.
If you regard Superman as impossible, then the only method that makes sense mathematically to maximize the plausibility of a Superman
story is to maximize the plausibility of everything except the single impossible element, or a select few impossible elements.
But this method doesn't produce the best stories,
if Superman is impossible, because it can lead to stylistic incongruities that are more of a negative than can be outweighed by what is at best an infinitesimal boost in overall plausibility
. The boost gained by this method is overall infinitesimal at best, because any gain is floored by the impossibility of Superman in the first place.
The way to get the most enjoyable story is therefore not strictly to maximize the plausibility of everything other than Superman.
On the other hand, if you are claiming that Superman isn't really impossible, so that the gains gotten by adjusting the plausibility of the other elements will be overall non-infinitesimally positive, well I think you've got an uphill battle to fight, to prove that Superman is possible.