I think a certain amount of dark plot elements are necessary. TNG's biggest flaw is that they were in a sterile plot bubble where nothing bad could happen ever and everything was perfect and would turn out just fine the vast majority of the time.
DS9 managed to find a great balance between being darker yet still maintaining the hope for a better future when the perfect society is conflicted with an imperfect world. Still, the show managed to have it's light hearted episodes.
Voyager too found a different balance. While they retained the "things mostly turnout alright" element of TNG and TOS, they had some definite dark moments though they were often removed by the reset button.
Things go wrong in real life and what makes stories interesting is how people deal with that. It's easy to be a "perfect human being" when everything goes perfectly after all.