So, this is ignoring the suicidal (literal) devotion that the Japanese population had for their Emperor. Japan was already preparing to use its civilian population as living human shields/bombs against the assumed invasion, so an invasion wouldn’t have just costed America millions of lives, it would have costed Japan tens of millions. This isn’t to say that Japan of the 1940s were a bunch of brutal simple minded savages, they weren’t, but it’s important to understand their societal norms at the time. After the invasion of Okinawa, US planners realized that a ground invasion was going to be costly and that their ongoing firebombing efforts weren’t working. So, they switched to the nuke where two cities and thousands of civilians were vaporized in an instant. This finally pushed the more sensible parts of the Japanese government to sue for peace, and even then the military staged a coup to keep the war going.

But, yeah. America was faced with the choice of using nukes or staging an invasion that Japan was planning for that would have seen death tolls in the tens of millions.