I would also assert that TOS humans were not some pinnacle of humanity. Kirk stated that "We're killers, but we're not going to kill today." Ie. TOS indicated that humans weren't perfect; rather they were always struggling to be better. In a sense, it was TNG that was unrealistic with the humanity perfected idea. The characters in Discovery and Picard feel more closely related to the people in TOS than later series in that they are not perfect, but constantly working to be their personal best. The hopefulness of that show, as well as of Discovery or Picard, is not that we will hit a magical end point where all our problems will be gone, but rather that we will grow into a society in which we can identify our imperfections and work to do better and be better today than we were yesterday.
And I guess that's why I prefer TOS and DS9 to TNG in that sense.