Many people have explored the "what ifs" of this event. Two novels stand out above the crowd. Downfall, by Maj. Donald Westheimer, and The Burning Mountain, by Alfred Koppel. Both these authors were personally familiar with the territory, having spent years in Japan, and having studied declassified plans for an invasion similar to Okinawa or Normandy. Both also compared the American plan with the Japanese defensive plans. Westheimer did the same for Okinawa and with the actual historic result.
Their projections were agreed that, most likely, the Americans would have had 500,000 to 1,250,000 dead and 3 million to 5 million wounded. The Japanese toll would have quite likely been about 9 million dead, with a likely 17 million wounded. They were scheduling mass suicide attacks including using housewives and children, with kitchen knives and sharpened sticks. Such things did happen on a smaller scale in Okinawa.
Among other results projected, the North Korean regime would have ruled all of Korea, and the Russians would have grabbed Hokkaido as a puppet state. My wife's father, due to be in the Downfall Invasion wave, might not have made it.
Given how the firebombings of Tokyo, Hamburg and Dresden produced much larger numbers of casualties than even Hiroshima and Nagasaki together, I will echo the quoted line: "Thank God for the atom bomb."