Either we go or we are likely to die out sooner rather than later and "we" includes all of the life on this planet. There are probably going to be 12 billion people, and maybe as much as 16 billion, on this rock before long and that is going to mean a much lower quality of life for everyone. We'll be drowning in our own wastes. If that doesn't kill us, sooner or later something catastrophic will hit us.
Beyond that practical consideration, humanity stagnates without a frontier.