The Israelites later did use Nechushtan (the Bronze Serpent) as an idol and it had to be destroyed.
God can heal in whatever way He wants, and we still have to obey His law. The purpose here aswell as healing was to be a type of Christ. (see John 3)
My minister recently preached on this, and pointed out that since the Fall through the Serpent (the Devil) the whole human race has been bitten and has the poison of sin coarsing through its veins. Our only hope for our healing is to look to the Cross, not a crucifix or two bits of wood, but the Cross as it is presented in the Bible.
Knox Free Church,
Perth, Scotland GB
His Name forever shall endure;
last like the sun it shall:
Men shall be blessed in Him,
and blessed all nations shall Him call (Ps. 72:17)