If the premise is that nuclear bombing was excessive compared to the firebombing and if the United States had waited a few more days between nuclear bombings they would have still continued with firebombing which was just as destructive as nuclear bombing and more people would have died.

The US wanted to end the war as quickly as possible and prevent the Soviets from invading and holding even a small piece of Japan for an indefinite period of time.

In doing so they probably prevented more deaths of Japanese civilians from continued firebombing and a Soviet occupation that would have been incredibly brutal.

And you can look at how the Soviets treated the people in the states that it occupied for decades after and compare it to how the US treated occupied Japan in the post war period.

The effects of both occupations still linger to this day, it's startling to look at economic differences in West Germany vs East Germany. It's doubtful that the USSR would have been able to prevent massive famines in Japan in a post war scenario where they occupied Japan or a portion of it.

A scenario where the US lacked nuclear weapons would have resulted in tremendous firebombing campaigns by both the Americans and the Soviets, as well as famine induced by a naval blockade and the war would have dragged on for much longer, drawing resources away from rebuilding both Europe and Asia, and the result would have been more civilian casualties from war and later famine that would have followed.

I don't think the blame for these civilian deaths in Hiroshima and Nagasaki falls on the shoulders of the Americans just like the deaths of civilians in the battle of Berlin were the responsibility of the European allies -- The blame lies squarely on the shoulders of the madmen who lead their countries to their doom for ultra nationalist reasons and a deranged sense of pride.

And that sense of pride thing there probably played into why the nuclear bombings were necessary -- The ultra-nationalist Japanese leadership needed to save face and it was easy to do so by surrendering to an enemy who had an unfathomably powerful new weapon.