Japan was pretty much defeated by the time we dropped the bombs anyway. We had been successfully bombing Japan's industrial cities/factories on the mainland as well. Wouldn't showing Japan we could obliterate an island have the same effect in delivering their surrender?
hi! check out this recent post for a quick summary & links to more posts. if you have follow-up questions, /u/restricteddata is your man
On the question of whether demonstration was a feasible, discussed possibility, I have written up a post on this question.