a horrible future that literally makes the whole show almost inconsequential (who cares what happens in season 5? The earth is going to burn so badly it will make Mad Max look like a hopefully future). I HATE stuff like that, with the future becoming so bad Earth reverts to the dark ages.
Not any one generation is going to solve all problems for all time. It would be silly IMO to insinuate that Sheridan made the whole Universe perfect. However, the fact that Garibaldi's mind was there to stop the bad guys 500 years in the future; and the fact that the Rangers still exist 1000 years into the future to help re-build the Earth shows that Sheridan and the Army of Light *did* have an effect on the future and they were definitely *not* inconsequential. (And the Rangers did have technology; they were just re-introducing it to the people slowly).
If the story took the tack that nothing bad would ever happen again it would be completely unrealistic.
ETA: Joe's comments
The fact, as I see it, is that no one and nothing will ever solve all of our
problems at once, now and forever. People will always be people. You can't
wave a magic wand and fix it all.
Yes, there was another war...but had the Shadows not been stopped by our
characters, there likely wouldn't have been a human race at ALL anymore.
Yes, there was a war, and many died in it...as tends to happen in war...but the
nominal right side in it came out on top, which would not have been the case
but for Garibaldi's simulacra giving them a leg up on things.
We have had, continue to have, and will always have wars, and grief, and
struggle...we will climb up and fall down...but each time we climb a little
higher, and in the end, we *do* build the world that our ancestors would have
wanted for us...we *do* leave the cradle at last, and we take our place among
the stars teaching those who follow us.
For my money, that's as happy an ending as we or anyone can ever hope for.