Comical and Strangelovian perhaps, but also the only hope for humankind. How many years until the expanding sun's corona envelops the earth? Assuming we make it that long without first destroying ourselves or meeting an unfriendly asteroid, you can bet that all of humanity will be behind the effort. So while it's not on this week's scrum board, it's definitely a high-priority TODO.
You make it sound as if humankind is doomed if we don't do it. But we're doomed anyway; no species on Earth has survived more than a coupla hundred thousand years without evolving into something else.
I think there's a desperate desire to live forever, behind some of the speculation about long-distance manned space-travel. Hey, guys, it's like this: you ain't gonna live forever. You won't be able to orbit Alpha Centauri, let alone colonize the galaxy.
100 years ago, we weren't at all sure that the Milky Way wasn't the same as the Universe. As we learn of faraway places, naturally we dream of visiting them; but now we know for sure that there are places we can see, that definitely can't be visited. We'll get over these dreams eventually, and maybe realize that we have to take due care of our home planet, and try to avoid killing one another in wars. Those are the great threats to 'hope', and the idea of survival through space-travel is a distraction for people who aren't prepared to face that fact.
So, so, so many years. We've gotten as far as we have in something like 200,000 years, with most of our technological advancement happening in the last...I don't know, let's be generous and call it 10,000.
It takes probably what, two hundred years for technology to become nearly unrecognizably advanced these days?
We have around 7 and a half billion years before the sun wipes us out. Trying to get people on Mars with current technology has its arguments, but that we'll run out of time if we put it off anymore isn't one of them.