The next time we hear anything about that brass serpent is in the 18th chapter of II Kings. They were even burning incense to it. It got so bad that King Hezekiah smashed the thing into little pieces. It had become an idol, just like the grotesque hawks and lions and serpents of centuries before. The means had become the end.
People do the same thing today.
They look at something that represents security and fulfillment, the kinds of things that a gracious God would be thought of as providing, and they begin to credit them with inordinate value. Not gods carved from wood or stone but big houses, flashy cars, a million shares of Microsoft, the trappings of success. Means and ends have become confused once again. And people make sacrifices of time, talent, treasure, and even the welfare of their children to those material gods in ways which would have made the ancient idol-makers think us just as absurd as we think them. In McGuffey's Reader is the old fable about the miser who had a sub-basement under his regular basement, known only to himself, where he kept his silver and gold.