An atomic bomb demonstration was feasible before the bomb drop on Hiroshima and Nagasaki. The problem is that the secret Manhattan Project would have been discovered if bombs went off in the New Mexico desert at the Los Alamos National Laboratory. Future tests were conducted on land and at sea, but before the bombs were dropped a demonstration wasn't feasible because the enemy would know what was coming.
As sad as it is to say I believe that a demonstration would not have done the job the way the actual bomb did. At this point in the war we were not going to be able to scare away the Japanese, we had to demonstrate our capabilities in a big way.
No one want to relive the catastrophe bombing of Hiroshima during the World War II international conflict. The atomic bomb is not a feasible option in regards to having a war weapon because of the long term affect of agent orange for an example that still ravages the American soldiers.
Yes, an atomic bomb demonstration was not feasible because a demonstration alone would not have brought the war to a conclusion. History has shown us in other conflicts, such as Vietnam, that calculated force designed to encourage the enemy to withdraw do not often work. With as persistent as the Germans were in World War II, anything but a strong show of force would likely not have ended the war. It was a difficult but necessary decision.
Infeasible simply means something is not possible to do easily or conveniently or impracticable. The problem with atomic bombs and nuclear weapons is that they are not impracticable. In fact, they are quite easy to use, but it is almost impossible to fathom the destruction they cause and the long term affects that they have.
The demonstration of the atomic bomb was not infeasible but countries were definitely not prepared for it. The US was very wrong in their actions and what may have won them a war 60 years ago has caused many issues ever since including now other countries catching up with the technology.
The bombing of Hiroshima and Nagasaki happened, and it was done as a demonstration to the Japanese that we could turn their islands into a smooth, glassy surface if they continued the war. So on those counts, I don't think we can call the bombing of those major cities in Japan infeasible.