Of course people can, and will. Humans have endured great hardships and mortal peril many times for political or religious or other reasons. For Americans, the Puritan emigration to a harsh new world is the most familiar. Hundreds of African migrants, almost all reportedly Christians, died last month when their boat sank during an attempt to escape to Europe. An excellent New York Times article last weekend described a harrowing crossing by refugees from Indonesia to Christmas Island; the reporter was told by Australian officials that "if we had left a few days earlier, the boat would have capsized."
The moon and Mars and the rest of the solar system can and will be colonized eventually (barring an existential event that Stephen Hawking and others have warned of). Yes, it's rather more difficult when there's no free atmosphere or radiation shielding. But it's a question of economics, not will, and every generation in the west is wealthier than the last.
The interesting thing, and this is what apparently alarms folks who would prefer HN to be Status Quo Anti-Innovation Political Curmudgeonly News, is that there's no guarantee that the political systems that emerge in the diaspora will be democratic. Some, I imagine, will be non-democratic and worse than our current system. And some will be far better.
Entire of itself,
Every man is a piece of the continent,
A part of the main.
If a clod be washed away by the sea,
Europe is the less.
As well as if a promontory were.
As well as if a manor of thy friend's
Or of thine own were:
Any man's death diminishes me,
Because I am involved in mankind,
And therefore never send to know for whom the bell tolls;
It tolls for thee.
Sounds like a species of massive assholes, and I see some ethical problems with letting them off their home-planet.
If you're serious about "ethical problems" letting our species of killer apes off of our home planet, how exactly do you propose to stop it? (Remember: Once you try it, you've just made these killer apes very mad at you.) Seems tricky, once the economics line up for space, unless you're talking mass genocide or mandatory doses of soma.
Transcending the millions of years of evolution that turned us into killer apes is the task of civilization and culture. But the megadeaths of the last century caused by wars between democratic governments actually dwarfed the prior century's wars between non-democratic governments. Maybe, to go back to the upthread discussion, there are better alternatives that don't involve the straw man of "isolat[ing] from each other as thoroughly as we can."