You don't leave the cradle beause the cradle is too dangerous.
You leave the cradle because it's too small and who other than a baby needs to sleep in something designed to stop a baby from wandering away.
Humans will leave Earth. These humans may not be members of such countries as the US or nations in Europe, because these nation may no longer exist by the time this actually happens. Perhaps, the citizens of Antarctic will be mostly the ones involved in this enterpise.
But opening the space frontier is something we could done decades ago and it's very unlikely it won't happen within say 5000 years.
We should do it now. It's morally the right thing to do. It's something that people living in a free society should do. And it's something that a totalitarian society can't do/won't do.
I also don't like the idea that we have to leave earth because living on earth will become in the future too dangerous- whatever that danger is said to be- whether nuclear war or space rock hitting earth [or a long list of other potential threats]. I don't like it because it's unrealistic and untruthful. And most importantly it suggest we going to "let" billions of humans die.
I think leaving earth to bring about a better earth- and one way is moving industry off world due to the space environment allows access to cheaper energy, and many other aspects which would be a economic advantage. Might be one good reason why one should open the space frontier.
Opening the space frontier will not "work" as some mega project which consumes large amount of tax dollars. Instead the emphasis should be to allow it to occur and pass specific laws that allow it to occur, and see it as an enterpise which will have benefits to Earthlings in general and the citizen of a nation in particular if they engage and encourage this reality to unfold- sooner rather than later.