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.
Then why were the civilians being trained to attack US soldiers with pitch forks? Why would I train my civilians to attack enemy troops in waves, if my goal was surrender?
That does not follow.
And again, I have no doubt that Japan was out of substantial military hardware. That wasn't even a question. In fact, that's part of the reason why they were preparing to throw civilians at military soldiers.
The problem is, you seem to be ignoring what happened on Okinawa and Iwo Jima. Which is... those Japanese troops fought without supply, almost no food, no water, and were coming out to our troops with nothing more than grenades often.... and they still... STILL fought to the bitter end.
So you can say they would have surrendered in a day or two without the bombs.... and that's a nice thought.... and you might be even right. It's impossible to say definitively what would happen in the counter factual.
Again, you have to look at it from the perspective of the people there, are that time. What we knew about the Japanese, is that soldiers literally starving to death, with zero support, and no supplies, were still fighting US troops for every square inch of Japanese islands. We had no reason to believe they were going to just give up. And every evidence that they would throw school girls with pitch forks at US troops, which would have been devastating to both sides.
'I Will Fight to the Last': WWII Japanese Soldier Diary, June 1943
Father repeated in his letter that I must fight to the last as an honorable warrior. I will fight to the last, always for the emperor. I will show them that we will fight to the last. March 6: “There is nothing quite so doubtful as to whether life or death be with one, yet we write at random like searching and traversing a battlefield.” March 31: “Pray for cherished glory.” My aged father wrote on April 12: “Even though your soul should remain in the South Sea Islands, follow the will of Heaven.”
Japanese soldiers diary.
Whether they would have surrendered or not, I don't know for sure. But every indication is that they had absolutely no intention of surrendering.