I agree that I'd like to see more hope and optimism as well, that is a staple of what Earth in this future is supposed to be about, its supposed to be a Utopia.
However, when things like the Borg, Dominion, and so forth constantly beat down on you, it does change your tactics and changes your optimism. If you were to do nothing, and be a complete space hippie, you'd die. The humans, in this future, are adapting to the environment around them, they are reacting, and they have to, purely to survive!
I think Star Trek itself, as a whole, has also evolved with the times, evolved with our real world issues and state of mind. I do think ST reflects our current state of mind, which is sadly a hardened look at the world around us.
All that said, I would like there to be a coming together of forces, an optimistic look at the future, but if there are really things like the Borg out there, how optimistic can you really be?