It doesn't mean you can't colonize Mars. It's just means babies born on Mars can't come to Earth. The centrifuge idea is one option. There is centrifuge city plan, better than a sky city plan. If it's stops going around, you've still feet on the ground.

You do know, the idea of colonizing means we will evolve to fill the other spaces, which means, by definition, long term goal is not to come back. Cloud city on Venus, well, that's just not sustainable. You're probably not smelting metals. You're reliant on Earth or Moon or Mars or Asteroid colonies sending you supplies. Like balloon replacement. If Earth fails, you're doomed. The reason we want to be at least a two species planet is so if Earth gets taken out, there is still people to rebuild.