When discussing this subject, I’ve learned to dismiss one particular type of argument out of hand. It’s the one that begins with some variation of: “My father/grandfather/uncle/etc. would have been part of the invasion force if the bombs hadn’t been dropped …”
I can understand the sentiment, but that’s pretty much all it is. You can take the exact same approach to question and undermine every military action in human history. How many Americans weren’t born because Abraham Lincoln couldn’t just leave the Confederacy alone? :-P
That's an excellent question and one I've asked before.
War is a brutal numbers game. Using the bombs was likely going to result in the lowest overall death toll and that's exactly what happened. I won't say it wasn't awful, but so is just about every other act of war. eg, its awful to stick your bayonet into some 18 year old kid's belly and watch him scream in pain and die in agony. The bombs were necessary and they ultimately saved a lot of lives. As such, using them was the moral thing to do.