That is why Mao organised the Chinese population boom. His plan was that in the even of nuclear war, even sustained strikes on Chinese soil could not kill everyone. Then in the aftermath they would still outnumber the surviving forces in the rest of the world, and could flood out to claim their lands for China.
His plan sucked for all the people he expected to die in those nuclear strikes.
Again, the survival-by-numbers purpose of colonies is a shit plan. We should have higher hopes for ourselves than merely survival by gaming the numbers. Do remember that those numbers are people, not resources to be spent and sacrificed. They want to live. They will have an interest in averting their particular disaster, and we should be working towards that kind of ability, rather than by hedging our bets.
Besides, the extinction of one colony will be little different to the extinction of them all. Communication between worlds is difficult. You couldn't have a conversation with someone on Mars, for example. You could send messages, but when the planets are near one another that is at best a twenty minute delay between replies, and at their furthest it's hours. Colonies in different systems entirely may as well be silent. Travel between them will also be rare within a system, and impossible for any real practical purpose between different systems. We can of course imagine some new method of travel that isn't limited by current physics, but that's little better than factoring a miracle into your calculations. Even the ability to colonise other worlds is unlikely to come with the ability to run and maintain a single interplanetary civilisation. Maybe one day. I think we'd be more likely to see an Independent Martian Nation than a Human Systems Alliance.
Any planetary colony is going to be genetically distinct from all other colonies due to the founder effect. It will develop its own unique culture in response to and in spite of the various unique challenges faced by human life on that planet, and the unique socio-cultural-historical phenomena that will arise there. They will develop their own scientific advances and technology, manufacturing processes, political systems, artwork and more. It seems likely that communicating much of this offworld will be prohibitively difficult. Given long enough, evolution will make its mark on the people and animals there in ways not seen on other worlds. All this will be unique to that one world, even though humans may exist elsewhere.
And the destruction of that colony is absolutely unacceptable. What can they do to prevent the loss of all that they are? Colonise yet more planets? That isn't going to do the trick. Those colonies will become unique just as theirs was. No. For them, Terra Nova must be where they make their stand, just as Earth must be where we make ours. If we colonise other worlds we'll need better reason to do so than simply finding spare baskets for our many eggs.
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