I think perhaps the Japanese had show willingness to a conditional surrender, but that the US would only accept unconditional. I believe that the US is the most uncompromising and dominant empire we've seen since Rome, and Hiroshima strikes me as a Carthage or sacking of Jerusalem like moment in establishing hiarchey as a global hegemon in a history defining way. In that sense it was necessary (from the point of view of a US imperialist) It was needed to break the spirit of the Japanese and subdue the entire region (even world) to US power. But in terms of being necessary in the strict sense of simply ending the war, you are probably correct in that it wasn't needed. Not sure if the "pax Americana" would have lasted without the psychological edge it gave the US for decades after, although the value of the so-called "pax Americana" is itself something that can be debated, I am aware.