An upgrade for ‘dead as a doornail’

I have used the expression “dead as a doornail” many times, and I am sure that you have, too. We are not alone. Since the 14th century, people have declared things to be dead as doornails. It makes sense that inanimate objects are dead, but then so are diamonds, doughnuts, and doorknobs. So, why do we use the expression “dead as a doornail?” ...