There are more risks than nuclear war. For example, asteroid impacts, pandemics, ecological disasters, etc. Even in the case of nuclear war, it's much easier t

There are more risks than nuclear war. For example, asteroid impacts, pandemics, ecological disasters, etc. Even in the case of nuclear war, it's much easier to detect and destroy an incoming nuke from another planet than it is to hit one that makes its transit in 5 minutes. Especially if information about the nuke is transmitted at the same time as the nuke. Also, once we make that first "leap across the black" to colonizing another planet, theoretically the next, further leap, will be easier/less daunting.