It’s generally accepted dude. By April of 45 their air defenses were about useless.
If they had such “huge” armies, and sent them at our positions, they would have to form up into large units to have any effect.
They would then have been destroyed from the air.
We saw the pattern on the other islands.
They dig out tunnels and deep fortifications. They take potshots at you until you destroy them with shelling and fire. They set up multiple defensive lines and successively retreat, you have to keep fighting them until they get to their final fortifications, then you burn/bomb them out. They have no way to get food into those final fortifications. They are military combatants, it matters not according to any sort of moral rules of war if you starve them out, as they can surrender any time they please.
Japan was withering into nothing by April of 45.
Anything that moved would have been blasted.
Could it have cost a lot of American lives ? It would depend entirely on how needlessly aggressive a manner the ground war was conducted. If you send thousands of men into overlapping fields of fire of well-trained and fortified gun positions with no body armor, it would be a slaughter. If you take your time, knowing that your enemy is trapped on their own island and they have no relief possibly on the way, you can mop up a lot more safely than if you push your men to run into their guns and take positions with bloody frontal assaults.
Of course wikipedia quotes other sources, which you can look up and debate whether you like those sources. It is generally accepted, of course, ss of April 1945, Japan was withering fast, and the buildup forces readying for an invasion were simply an act of desperation. IMHO, if an invasion was simply postponed until 1946, and Japan was thoroughly blockaded for the winter of ‘45-’46, it would have “softened naturally” to a significant degree. It’s not like they were going to “get away”. This would also have allowed for all those months of continued war production and preparation for the invasion, and putting into service even more naval and air forces. I doubt that surrender would have even taken that long. But the final scene in the heroic movies would have to be Japan simply surrending instead of a climatic battle, or in the actual case, the atomic bombs. The real goal of the elites all along was putting nuclear weapons into active military service.
By the lefties that edit Wiki. The occupation force found a different story. Your way means the POWs die. Fixed fortified defenses like in Iwo were impervious to bombs and shell.. They Japs didn't have to attack, just defend, just like Iwo. 21000 Japs accounted for 26000 Americans. The casualty estimate was likely too low. And food water are pre-positioned in defensive positions, the Japs knew about logistics too. It was the proximate cause of the war absent Jap aggression in general.