- almost no chance of ever finding life on other planets
- we'd have to take better care of the earth I guess; even if we colonize the moon and Mars, that's only another 0.35 earths worth of surface area. We'd run out of rocks to live on, and soon.
- things wouldn't seem so boundless. You wouldn't get to look into the night sky and pretend you're going to visit all those stars, like Han Solo.
- speaking of which: would we still have sci-fi with many different star systems? or would that idea just be inconceivable, or hard to visualize, kind of like sci-fi over many different universes is now?
What if the Earth were the only planet? We'd be done: all the space on the planet would be all the space there is! There would be no more anything! How would that affect the way we think?
Don't worry, I'm not trying to make a point here. Just try thinking as if that were true for a minute; it feels weirder than I would have thought.