If the A bombs failed Lemay was armed and ready to launch an aggressive incendiary bombing program which would have killed MILLIONS.
The reason they were using incendiary devices almost exclusively over Japan was because the air currents from Asia did not allow bombers to accurately drop conventinal HE bombs on target. The AAF went to the incendiary devices because accuracy was out of the question due to prevailing air currents and since much of Japanese industial zones (and the cottage war industries) were in and around cities constructed mostly of lumber, incendiary methods were deemed the most effective.
The United States was going to defeat Japan by simply burning it to the ground, the ethics of the act be damned and arguing about whether atomic bombs were justified is nothing more than a distraction. People were burned to death for months before these bombs were dropped and how that could be acceptable because conventional weapons were used is simply absurd.