I should clarify that when I wrote plausible I meant the societal aspects you touched on in your second point. As long as we are human, we will continue to experience our imperfection and continue to be prone to giving in to our base desires. And of course one group's culture shapes their ethics differently than another group's. Ethics is, after all, arbitrary. But one could still look at Star Trek and imagine a society where people's needs are met better than they are now, where we have systems in place to prevent a complete erosion of society better than we have now, where the basic principle of not exploiting others for personal gain is taught better than we teach it now, where we don't tolerate those that violate that principle.
You will still have hate. People will still kill. But one could look at Star Trek and imagine it's plausible to live in a society that doesn't tolerate school shootings or the systemic breakdowns that led to that.