I think this ignores the historical context. The US had air control and been bombing the mainland relentlessly for a while. The firebombing of Tokyo had already caused bigger loss of life than the Hiroshima bomb did. So it was clear to both parties that the US did have the destructive capability and were willing to use it. This did not cause the Japanese to surrender. Why would it have made any difference to "demonstrate" the bomb, if actually bombing the capital and killing 100000 did not have that effect?

Even after the second bomb, just before announcing the surrender, there was an attempted coup d'etat by high ranking officers to prevent the surrender. So its not like surrender was a given after seeing a nuclear bomb.

It is only in retrospect it seems obvious that the nuclear bomb would lead to surrender. Even taking about "the decision to drop the bomb" as it was some unique moral dilemma is kind of anachronistic. Of course they were going to drop the bomb, just like they used any other weapon at their disposal.