This is pretty much anathema to the sort of thinking that makes humans worthwhile.
Maybe it isn't possible to get the human race away from this planet, but if it is, it's literally more important than anything else we could do. What's in the best interests of the human race is making sure we can't simply commit global suicide, which we're very much capable of doing right now. Get us off the planet and surviving somewhere else, and you've more-or-less guaranteed us an eternity in which to figure out how to be more decent beings.
Why is the child's dream not the one that thinks we're going to achieve peace on Earth before we find a way to get off the Earth?
I simply have more faith in our technological prowess than in the depth of our morality/common sense.
What would you propose as a more realistic goal? Peace on Earth? An end to war? Respect for our environment? Nuclear restraint?
To me, those sound like a wishful thinking.
Human nature has seen us killing one another for stupid reasons for the entirety of out existence. We've already committed to a path, and if peace on Earth was our priority we'd never have left our caves.
Take the best we have to offer, shoot them into space, and preserve the best of us for eternity.