What is the main idea of "By the Waters of Babylon?"
In my opinion, the main idea of this story is that science and technology are a two-edged sword. They can help us, but they can also do great harm to us.
We can see that John's people would in some ways be better off if they had science and technology. They would have metal and canned food and stoves that use electricity and indoor plumbing. All of these are things he sees in the "Place of the Gods." His people can not make these things.
But what will happen if John sets them on the way to technological progress (it looks at the end like he's going to try to do this)? Will they be better off, or will they end up destroying themselves like the people who destroyed what used to be New York City?