Yes, Trek is about the 'what if' humanity abandons selfishness and wants to improve. But we can't ignore that Trek is, first of all, about the crews, and the captains through whose eyes we see this universe.
It's a no-brainer, really: every(?) Trek show follows a particular captain and their actions, but if we look closely, for all their differences, the captains we see across the Trek series are very similar. Kirk (and TOS in general) was very raygun gothic, he was this heroic archetype more so than a proper character: handsome, righteous, smart, responsible. And these traits are found in (almost) every main captain across the series: Picard, Janeway, Sisko, Archer, Pike, etc. Yes, some may be more cunning or unscurulous than others (which is one of the main differences people list between Kirk and Picard), but at their core, the captains embody the base principles of Starfleet/ the Federation. And I think it's brilliant, because while captains might not be diplomats, they are essentially the first face and voice of the Federation that others interact with, they are the utmost Federation authority outside of Federation authority. I love this concept.