It's the broad brush that gets me. Going dark has to be a principled choice. DS9 nailed it with the "it's easy to be a saint in paradise" mentality. The hope in most Trek writing comes from a worldview that most people are inherently good unless their environment forces them to act otherwise (which I would very much say is how the world actually works). When shows go dark using something closer to a "people are inherently evil unless they really fight back against their nature" mentality, it feels bad and rings hollow.