The money funding SpaceX could hardly be allocated towards sustainable colonization of Mars in a more efficient manner. If you can think of one, please email Elon.
Sure, SpaceX has good business plan. That's the point. Building rockets only for a future Mars voyage and not making any money along the way would be a bad way to do it.
By contrast, fusion will only be a scientific research project for many years and many billions.
By contrast, fusion will only be a scientific research project for many years and many billions.