If a similar debate ever arises I offer a practical solution. Ask everybody in the country the question. Draft those who give the anti-Truman answer, using the age/gender limits the Japanese historically followed in their home island defense preparations. Use them as your invasion force. If/when they fail you may try the Truman option. The eventual winners will be better for the process.
The argument that "the U.S. had to drop atomic bombs on Japan because an invasion of the Japanese mainland would have cost [thousand/millions/etc.] of lives of U.S. military personnel" is predicated on the assumption that an invasion of the Japanese mainland would have been necessary, or even a legitimate option at all.
I contend that there is a very unhealthy relationship between a citizen and his government when you have millions of servicemen who are willing to engage in this kind of military action just because some @ssholes in Washington, D.C. think it's a good idea. There is no principle of liberty in a free nation that would ever compel someone to follow orders like this without even being reflective about what was really at stake.
There's a reason why the same "Greatest Generation" that fought World War II oversaw the subsequent military debacles in Korea and (even worse) Vietnam, and was part and parcel of the moral and social collapse of America that began in the 1960s. Most of the leadership from that "Greatest Generation" was anything but.