Possibly, but it seems speculative to me that this was an important thing. I imagine it would have been very surprising and shocking to have a demo nuke dropped

Possibly, but it seems speculative to me that this was an important thing. I imagine it would have been very surprising and shocking to have a demo nuke dropped a few miles away from Tokyo. Maybe less shocking, and that difference in shock value reduces the probability of surrender by an unacceptable amount?