At the end of the day Earth has a thermodynamic budget in order not to experience run away heating. Moving to things like fusion may delay the global warming caused by carbon, but our hunger for more energy will eventually lead to energy use beyond what we can radiate away.

Hopefully, 1, we get to the point we have to worry about that. And 2, we'll have cheap and easy access to space where we can move entropy intensive processes away from the planet.