Well, it certainly was, and for the reasons I said:
(2 Ki 18:3 KJV) And he did that which was right in the sight of the LORD, according to all that David his father did.
(2 Ki 18:4 KJV) He removed the high places, and brake the images, and cut down the groves, and brake in pieces the brazen serpent that Moses had made: for unto those days the children of Israel did burn incense to it: and he called it Nehushtan.
Once again, you erred because you dont know your Bible.
Note, you said God would have destroyed it before it became an idol. Then, by the same principle, God would have destroyed the serpent before anyone thought of worshiping it. Instead, it was destroyed by Hezekiah afterward.
I was hoping you'd be smart enough to realize the distinction yourself.