Space colonies are not really optional if humanity wants to continue expanding. The total land area of Earth is approximately 150 million km². Mars is smaller but has no oceans so the area there is about the same. This is a lot but it is very much finite. As a comparison, Ceres, the biggest of the asteroids, contains enough material to construct perhaps a trillion of O'Neill's K1 Bernal spheres with a total living area of hundreds of billions of km². This is vastly more than we can ever hope to find on planet surfaces. And this only from one single asteroid.
Space colonies are not really optional if humanity wants to continue expanding. The total land area of Earth is approximately 150 million km². Mars is smaller but has no oceans so the area there is about the same. This is a lot but it is very much finite. As a comparison, Ceres, the biggest of the asteroids, contains enough material to construct perhaps a trillion of O'Neill's K1 Bernal spheres with a total living area of hundreds of billions of km². This is vastly more than we can ever hope to find on planet surfaces. And this only from one single asteroid.