You're not alone
FINALLY, someone agrees with me on this!
What the soviets did was show the japanese army how badly prepared they where for mechanized warfare in open ground, as opposed to holding out on small island fortresses. Casualty estimates for an invasiont of Japan should be based on the soviet experience in 45, not on Iwojima experience.
Even in a country with difficult terrain as Japan, if you have the room for it, it´s manouver warfare you're talking about, not island assault or jungle fighting, and for that the IJA was not ready, and the Red Army demonstrated that brilliantly.
The US might not have the Soviets expertise at large scale armoured battles (having never done anything has bold or big as Bragation) but they where getting better at it and where the kings of tactical air support.
Yes there would have been lots of casualties if the allies had invaded Japan, but they would be mostly japanese casualties...