I'm also curious as to why what the US did to Hiroshima is an unthinkable nightmare scenarios whereas what the US did to Tokyo is not (and nor is what the US did to 71 other cities in Japan also not that).
Dead is dead.
You are correct in the sense that Japan was going to surrender at some point, whether that was due to conventional bombings, starvation from naval blockades or an actual invasion by the USA, there's no scenario where they don't sign an unconditional surrender.
Most of those scenarios would have killed more people than the two nuclear weapons being dropped.
Say what now?
Tokyo was bombed by the United States Army Air Forces (USAAF) ..
The strikes conducted by the USAAF on the night of 9–10 March 1945, codenamed Operation Meetinghouse, constitute the single deadliest and most destructive aerial bombing raid in human history.[1] Sixteen square miles (41 km2; 10,000 acres) of central Tokyo was destroyed, leaving an estimated 100,000 civilians dead and over one million homeless.
> Do you see a difference in razing a city with one bomb in an instant or with thousands of bombs
Tokyo vs. Hiroshima - https://blog.nuclearsecrecy.com/2014/09/22/tokyo-hiroshima/
Do you?
(Aside from thousands of conventional bombs being significantly cheaper and lower tech than a nuclear bomb).