Being able to "hack" gravity - some form of mass reduction field to mean you can get off a planet with less energy, and avoiding the rocket equation, would be transformational. Either way, assuming Musk doesn't run out of money before delivering some financial success, I see humans reasonably occupying the moon/lagrnge/asteroids/mars for good in the next 50 years. Once that happens, we'll be on the way to self sustaining habitats. Assuming we get Fusion in the next 100 years energy in the outer system will be fine. With population growth in space then surely at some point there will be those aiming to leave the solar system. Accelerating to 10% light speed to emmigrate to other systems will take a hell of a lot of mass. Without FTL travel or at least FTL communication, human civilisation will diverge if we leave the solar system.