It would have been costly to invade; by the end of the war they had almost no fighters left to shoot down our bombers and they still fought on. As I understand it, most Japanese had no idea that the damage from the first bomb (if they were aware of it at all) was caused by one bomb from one plane. Even if they had known, there was just something fanatical about their culture that would have prevented them from doing anything but what their government ordered.
They were fighting for their living god...the emperor
The casualty and death estimates for the US were over a million soldiers. It would still be going on