I for one, based on what I've read, and first hand accounts of Japan leading up to, and during WW2... am I absolutely convinced that millions would have died if Japan had refused to surrender, and we had landed on the mainland in a state of war. Millions. Guaranteed.
That's nice.
The reality. Japan was out of war materials, most of their Navy was sunk, they had expended all their aircraft as Kamikazes to very little effect, and most of their experienced Army divisions were trapped in China.
SOOOOOOO, what they had were civilians with pitchforks. They were desperate to surrender, and that's what they did once the USSR got into it and they had no chance for a negotiated settlement that allowed them to keep some of their colonies.