I doubt we'll ever get to that point. Keep in mind it's easier to colonize the ocean floor or put domes over Antarctica Than it is to go to a whole other planet and set up shop. Both options are even easier and cheaper than terraforming the moon.
To put this in perspective, You could glass the entire surface of the Earth with fusion bombs, And the remnants of the planet would still be more habitable and easier to get to than mars. Especially when you factor in that most of the underground is unused.
So unless one of these scientists at harvard creates a black hole that eats the entire planet, its highly doubtful that we will have to leave earth. Its more of an option , than a requirement. Even silly things like global warming or super viruses are a non-issue when compared to the hellscape that is mars or titan or io or god forbid venus. And no, You can give up on going to other solar systems cause that will never happen.
