Well, it wasn't me saying that. However, it's worth considering that the US only had about 90,000 soldiers killed in France and Germany from D-Day through to the end of the war, and while they were only facing about a fourth of the German military (the rest being occupied with the Soviets), that still represented manpower greater than what Japan had available with most of its army being trapped in China. And Germany had a still-mostly-intact industrial base more than capable of equipping its troops with as much modern weaponry (guns, artillery, ammunition, tanks and armored vehicles and airplanes) while Japan's industrial base (which had never been anywhere near Germany's in terms of productive capacity to begin with) had been smashed almost to nothingness. Schoolgirls with machine guns (and very little ammo) have much less military effectiveness than perhaps people imagine.

If 50,000 casualties would have proved to be an underestimate of the cost of an invasion, it likely would have been the result not of angry common Japanese armed with sharp sticks and fighting to the bitter end, but of the 6,000 to 10,000 planes the Japanese had amassed and hidden away for use as kamikazes. These piloted bombs (which were really one of the most devastating weapons of the war) caused considerable carnage despite the US' air supremacy; unleashed against large troop transports carrying thousands of soldiers each which of necessity would have had to have come very close to the Japanese coast, they might well have killed a lot more than 50,000 soldiers.