Room Enough, And Time
A Return to Madness: Nuclear proliferation leads to an accelerated nuclear arms race, and eventual widespread nuclear destruction.
The Golden Age: Humans overcome the major problems of religious and ideological conflict. Most causes of premature death are controlled. People learn to be happy with what they have and stop striving for the unreachable.
The Wheel of Time: The odds are that every civilisation will fall. Disasters may happen that will throw humans back to subsistence level. The return of the cave man, with superstition replacing science.
In the end, Vinge joins Stephen Hawking, Gerard O'Neill, Carl Sagan, and other intelligent students of the human condition in suggesting that off-planet colonies, outposts, and generation ships may be the best long-term chance for humanity's survival.
Why not solve the problems of humanity on earth, before setting out through more hostile territory for "castles in the sky?" The sad truth is that humans must work both ends of the problem, to maximise the chances for long-term success.
Human beings have an average IQ of 90 at the current time, with a strong likelihood that the average IQ will drop to 84 by the latter half of this century. Some countries are fortunate enough to possess concentrations of individuals with higher intelligence, along with good intentions toward the rest of humanity. This fortuitous situation will not last forever. We had best use the good minds while they last--to create better chances for survival both on earth and in space.
I will leave you with five graphic disaster scenarios to ponder. Remember, these are not the worst that can happen. Enjoy.