Possibly someone has a viable reason to let the Earth be destroyed and gain something better and greater instead. I am not aware of any reason like that. All we have at the moment are just hopes to find other planets suitable for our form of life. We are still speculating if there is life on the nearest planets. We have never been there – we have only started selling space there. (Who is going to buy a property that he has no way to use in the nearest future?)
Let's say we know a good planet and also know how to get there in 10 years. We immediately send a hundred people to colonize it just in case we will have to loose the Earth. But who knows if there is a factor making it impossible to beget a functional human being outside of Earth? Who has the assurance that sending people to colonize outward space is not sending them to die out there without any hope of reproduction?
We are sending people to space for decades already. What are they doing there? If they are not doing that one thing they are supposed to be doing, they are spending Earth's resources for nothing. I want to hear that two young people conceived and born in space having a baby in space. News like that will prove me that people can fly away from our planet to populate the Universe. But what are they telling us now? They are reporting some half way experiments with rats!
http://curious.astro.cornell.edu/question.php?number=702
They don't even report on attempts to reproduce humans outside of Earth. Unless we find out that it is possible we should make every effort to preserve the Earth's environment to be able to stay here as long as we need. We should not endanger our cities with nuclear power plant explosions in particular. Now at the time of solar energy breakthrough we can almost freely collect electricity from the only God given safe nuclear power plant – the Sun!
Our Earth is a unique place where humans ever reproduced themselves. There are so many people in the world that we forget about importance of ability to reproduce ourselves. Without such an ability all of our billions in 90 years will come to zero. We need to stop and think about better ways in life and particularly in energy safety. Solar and wind energy are considered renewable and clean sources of energy because they don't use up or endanger our nature. We are all responsible to make our own transfer to the clean energy for our homes. This is one of the ways to preserve our planet, which we all need very much.