I agree with most of what you say but the American people were tired of war in 1945. Also the only way that we could have defeated Communism would have been invasion of both the USSR and the Chinese mainland.
A quick look at history shows that there haven't been many examples of success doing that
Would the world have been better of if both Stalin and Mao were defeated? Absolutely but it was going to happen then. At the end of the war we had a total of 2,000,000 serve in the European theater. The Russians had 29,000,000. Subtract casualties and they still had well over 22,000,00 under arms in 1945.
Well, China was in Civil war.. we would have just needed to tip the balance to Chiang Kai Shek and China would have been spared Mao's purges.
I did mention the desire to fight in my previous message. You are correct, it would have been a tough to impossible sell. As for numbers, we would have had to turn out the Europeans to fight against their occupiers.. including Germany.. yikes. There might also have been plenty of Russian who might want to fight against Stalin and his oppression as long as it wasn't to become slaves to Germans.
It was not something we could do all of a sudden at the end of the war. We would have had to have planned for it all along. And that probably would have included making a choice as to how far we'd help Russia defeat Germany.
I'm thinking now of the Iran-Iraq war. We were okay with them killing eachother.. but we wanted neither to lose. Had one side been victorious, two moderate problems would have become one HUGE problem. Maybe that is something we learned from the end of WWII.
As for the general topic of casualties and so on.. don't forget the firebombing campaigns did a lot of damage. The Tokyo firebombing devasted a city made largely of wood. Killed over 100K people. Dresden maybe 25K. I remember reading about "bat bombs" which were never deployed but were tested and were effective. The USA was going to try it against Japan. And Japan used balloon bombs.. hydrogen balloons carrying incendiaries where they hoped the entire forested northwest would be set ablaze... and later that wanted balloons to carry over THE PLAGUE.
Nuking them was reasonable.. not very nice.. but reasonable.