Nuking cities or conventional bombing them is the same thing. The bombing of Tokyo resulted in about 100 k deaths https://en.wikipedia.org/wiki/Bombing_of_Tokyo

Only the emotional impact was different. A nuke was an otherworldly weapon that marked a technological superiority that was impossible to overcame during the war, hence the surrender. Carpet bombing would have to go on until enough Japanese cities and industries were reduced to hash. Then maybe an invasion. As somebody pointed out, the surrender of the Japanese troops in China was a goal of the war and they could not carpet bomb or nuke China.