You're right that 70ly is a long, long way. But three things counter that:
1.) Suspended animation is getting closer to real all the time and most of the supposed problems with that (such as radiation exposure in transit) are actually just matters of cost, not absolute limits.
2.) Generation ships.
3.) Yes, but what about getting to the nearer stars in part in the knowledge that the next generation would take the next step further away?
Perhaps the first two are examples of what you meant by "survive in space for a long time".
And keep in mind that what we can already see of a solar system is very far from complete. We may not be able to see anything orbiting Alpha Centauri yet but our resolution is rough indeed. In fact, our current techniques are in large part not even direct viewing but just imperfect means of derivation. I think that it's safe to say that we would find something there beyond just a ball of plasma.
As for your "they would go mad" absurdity, citation please. People have gotten by without contact with "civilization" for years on a regular basis for most of history. Look into how most of the Pacific islands were first settled. Or at some of the long duration nuclear sub cruises, which were shorter but still in very cramped spaces. Just bloody well watch Master and Commander or anything decent about 1500's to 1800's sailing ships.
People are tough. They adapt. Many of the proposals for ships to other solar systems have long proposed crews of several dozen or even more, quite enough to create a small society of their own. And if we were to choose to do it that way, we could send ships in clusters so that they would not all be at risk from one point of failure but could still communicate across the ships in transit and know that others would be there by the time they reached their destination.
Not only that, there are always some people out there who find the idea of near total isolation a feature rather than a bug. Just look at the history of forest service fire watch towers or lighthouse personnel. The travelers have plenty of ways to deal with human factors. Hell, maybe some of them would use up most of the trip smoking pot and playing video games. Add in a "real doll" and plenty of people would find it an improvement on their current level of social interaction. And if you think that I'm joking you're not paying attention.