I read your post. You said you wanted a different world (hence the new planet), and no current cultures. 140 years in the future, there will be plenty of cultures that don't exist now, just like the hippies didn't exist during the civil war. That would give people the ability to use a modern culture if they were set on it, but make up a new one if you wanted. I thought it was a decent compromise.

The "move into a barren wasteland" thing is a good idea though. I just didn't want it to turn into fantasy (ie magic), as it's a bit easy to deal with problems like that. A different Earth with humans (or some other race) on it might work, though.