The atomic bombs were devastating and impressive, but they didn't do much more actual damage than the B-29 fire bomb raids on Tokyo in terms of loss of life and property. When you can drop bombs at all, it doesn't really matter what kind of bombs they are. The atomic bombs were an excellent PR move of sorts, as a tool to convince the japanese to give up more than as a weapon of war.
I think that dropping incendiary bombs on cities made mostly of wood and paper is even worse than dropping an atom bomb. And it wasn't just Tokyo; dozens of japanese cities were burned to the ground.
You're talking about two bombs on relatively unimportant targets, specifically because they wanted relatively untouched cities to erase as a demonstration. That won't kill as many people as years-long bombing campaigns on more populous cities. They were staged demonstrations, not attempts to kill massive amounts of people.
If Japan didn't surrender after the first two atom bombs, the next half dozen would have undoubtedly caused much, much more damage.
