This is a perfectly legitimate what-if hypothetical.

Aside from several points already raised by others (some accepted by the author), I see two others. First, the more obvious one. Jews and others were being gases and incinerated at an horrific pace. If D-Day had been delayed, the concentration camps would have completed the Final Solution, i.e. the camps would have been empty when "liberated". (The Japanese weren't exactly easy on their captive populations either.)

Second, not one commentator seems to know about the minutes of the Japanese War Council that was convened immediately after the Russians (as promised in Potsdam) entered the Pacific Theater and proceded to rout the Japanese fom Manchuria. On August 9th, the Emperor participated in the Council for the first time ever. He had a list of failings of the Imperial Army, upon which Hiroshima featured, but not prominantly. (Keep in mind two dozen Japanese cities had already been obliterated by firebombings, several of which turned into firestorm, killing 100,000 people at a shot.) When news arrived from Nagasaki, the Council breifly took note and then return to the major items. In short, this had NOTHING to do with a "one-bomb" war.

"But..but.. the Emperor said he was surrendering because of the bomb!!" Dig a little deeper. The Imperial Army was on the verge of open rebelion at the idea of surrender, the most dangerous thing the Emperor could do is blame them for the terrible defeat. The Bomb provided him with the perfect face-saving way to explain the surrender. As it was there was an attempted military coup, but key military leaders refused to join it. The speech the Emperor had pre-recorded was known to at least some of them and made it possible for them "to bear the unbearable; and suffer the insufferable."

If Japan (or Germany) had been left in "peace" for the years while the Manhattan Project plowed ahead, how would it have looked in world opinion for the US to then start incinerating their cities, spewing radiation all about and, if more than a several dozen cites were turned into firestorms, be held accountable for triggering a worldwide nuclear famine? (Which would have come as a shock then as well as any time up to 1985 when atmospheric scientists first elucidated it.)

The bottom line: the Bomb is illsuited for use by a civilized army, its only reasonable function is to deter use by barbarians. (And not really even that; but that's material for an essay I plan to write soon.)