This is the same false dichotomy that gets floated in every single discussion of America’s decision to use the atomic bomb; either mount a mainland invasion of Japan, or force them to surrender with the overwhelming force of the atomic bomb.
The reality, of course, is that there were any number of alternatives. A lot of historians think securing Japan’s surrender may have been as easy as agreeing to let them retain the Emperor earlier (which they ultimately did anyway.) The 1946 report of the US Strategic Bombing Survey found (emphasis mine):
Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey’s opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated.
You can say that it’s not fair to judge history, which carries some weight, but a lot of people contemporary to the bombings had doubts about it at the time that were never addressed. Truman never even consulted MacArthur about it, even though he ran the Pacific campaign, ostensibly because of how much the two hated one another. MacArthur’s biographer reported that had he been asked, he would have opposed the action as unnecessary.
It’s a tremendously complicated question — whether or not the bombings of Hiroshima and Nagasaki were justified and/or necessary — and I hate to see it so consistently reduced to worthless, binary logic. The most odious commentary on the whole thing I think came from Truman himself, who said that he never lost a wink of sleep over it. Even if you had utter confidence in the weight and rightness of your conviction, I think it says something pretty contemptible about you to not lose sleep over condemning hundreds of thousands of people to painful deaths.