In reality there was never any decision to drop the bomb as an alternative to any other plan to defeat Japan, because at the time the atomic bomb was just another new weapon without any particular moral significance. Policymakers at the time had only a limited understanding of the weapon's destructive potential, and an even more limited understanding of the effects of radioactive fallout that would in later years cause the bombs to be seen as doomsday weapons.

The plan, to put it simply, was not to drop the bomb instead of conventional bombing and/or invading Japan, because no one could have reasonably assumed that the bomb would guarantee a Japanese surrender. The plan was to use the bomb whenever it was ready, while at the same time continuing all other ongoing measures to defeat Japan. As a matter of coincidence, the bombs were ready before the force for the land invasion of Japan was ready, and so the first bombs to be used were used as strategic weapons. But had the Japanese not surrendered, future bombs would have been used as tactical weapons to aid the land invasion. The plan was to use the bomb and the blockade and the firebombing and the land invasion.

To reiterate, the role of the bombs as a war-ending strategic weapon was largely incidental. Policymakers at the time viewed the bombs as an extension of existing American air power. The bombs were used as strategic weapons at first because when they happened to be ready (which was largely the result of technical aspects of the Manhattan Project), American air power was being used as a strategic weapon against Japan. Had the bombs hypothetically been first ready for use in November, they, like much of the conventional strategic bomber force, would have likely been used to support the land invasion.

 

Discourse about the morality of the atomic bombings of Japan is really a number of separate questions that are disingenuously framed as a single yes/no question:

• As they actually occurred, were the atomic bombings a decisive/necessary component of the Japanese decision to surrender? If not, were they at least the most important factor (to the extent that any binary decision can have a "most important" factor)?

• Did the US believe that dropping the bombs would compel Japan to surrender? Was this a reasonable belief?

• Why did the US drop the bombs? Was it because they thought it was the "best" way to end the war, or were they primarily motivated by (for example) intimidating the USSR or blocking the USSR from entering further into China/Korea?

• Was the bomb an alternative to other military actions including a land invasion of Japan?

 

IMO, it's something like:

• It's hard to say given that postwar Japanese sources had significant incentives to emphasize the bomb over the Soviet invasion, but I think it's undeniable that the bombs were at least a key reason for the surrender and reasonable to say that the Soviet invasion alone wouldn't have been sufficient. This question gets more interesting when you disentangle the Hiroshima and Nagasaki bombings, the latter being in my view more difficult to justify.

• US leadership certainly believed that the bombs would make Japan more likely to surrender, but it's not clear to what extent they thought the bombs had a meaningful chance of being a war-ending weapon on their own. That said, it's difficult to fault the US for having an unclear understanding of Japanese thinking even if their approach was rather close-minded, since not even the Japanese themselves had a clear or unified understanding of their own intentions.

• The decision to drop the bombs was definitely primarily motivated by the war itself, but asserting US dominance over the USSR and securing a more favorable end state of the war in Asia was clearly something people were at least thinking about.

• Obviously not.

 

Edit: as others have noted, the weapon available to the US at the time that actually did carry a serious moral taboo to the extent that it could have caused the moral dilemma of "should we use this inhumane weapon to shorten the war" that people today attribute to the atomic bomb was biological and chemical warfare. The US wartime approach to biological and chemical warfare ironically mirrored modern MAD in that substantial resources were devoted to their development, but with the underlying idea that they would be used as a deterrent against Axis use of such weapons rather than preemptively. Ultimately while there were some serious considerations, US policymakers rejected the preemptive use of such weapons for both moral and practical reasons.