I for one still don't understand how a story about rebuilding utopia cannot be optimistic and idealistic, especially if we get to see Discovery and its crew serving as the spark to relight the flame of the Federation from its embers. I've long believed that this kind of message is actually what today's audiences would need and appreciate; if I think back of how I thought about TNG over the years, its utopian message of how humanity collectively solved all its problems and conflicts in the backstory and now Earth was a joyful, Kumbaya-singing paradise gradually changed from a positive 'we can do better, there's hope for a better future' to a downright unrealistic escapism. Right now, I would think, it's about time we are given something that actually tells us that no matter how bleak things seem around us, even if we think all efforts to make the world a better place have already failed, there's still hope and don't you dare believe you're not enough to contribute to it, because small individual efforts add up to become collective tidal waves of change.
From what I'm seeing in the trailer and based on what Kurtzman said at the panel, I'm looking at it like this:
Perhaps in this future, the Federation and Starfleet have sort of lost their way. It's not the Federation that we, as the audience, or the crew aboard Discovery remember. Discovery, being that beacon of hope, serves as a spark for the Federation of this era to remember who they once were and how they can be better. We already saw that in the first two seasons where Michael and the Discovery crew constantly remind each other, or Starfleet, what Starfleet and the Federation are.