Marley was dead, to begin with. There is no doubt whatever about that. The register of his burial was signed by the clergyman, the clerk, the undertaker, and the chief mourner. Scrooge signed it. And Scrooge's name was good upon 'Change for anything he chose to put his hand to. Old Marley was as dead as a door-nail.
Mind! I don't mean to say that I know of my own knowledge, what there is particularly dead about a door-nail. I might have been inclined, myself, to regard a coffin-nail as the deadest piece of ironmongery in the trade. But the wisdom of our ancestors is in the simile; and my unhallowed hands shall not disturb it, or the country's done for. You will, therefore, permit me to repeat, emphatically, that Marley was as dead as a door-nail.
So begins Dickens's A Christmas Carol, and while we best remember the expression "dead as a door-nail" from this introduction, Dickens makes it plain that the doornail predates him as a thing than which other things are as dead as.
So why a doornail?
Merriam-Webster doesn't even make any bones about it, defining "doornail" as "a large-headed nail - used chiefly in the phase dead as a doornail."
World Wide Words dates the phrase to at least 1350, and examines how it came to be:
The usual reason given is that a doornail was one of the heavy studded nails on the outside of a medieval door, or possibly that the phrase refers to the particularly big one on which the knocker rested. A doornail, because of its size and probable antiquity, would seem dead enough for any proverb; the one on which the knocker sat might be thought particularly dead because of the number of times it had been knocked on the head.
But William and Mary Morris, in The Morris Dictionary of Word and Phrase Origins, quote a correspondent who points out that it could come from a standard term in carpentry. If you hammer a nail through a piece of timber and then flatten the end over on the inside so it can’t be removed again (a technique called clinching), the nail is said to be dead, because you can’t use it again. Doornails would very probably have been subjected to this treatment to give extra strength in the years before screws were available.
Screws? No way! You mean it's not nails anymore? I'm going to look into this right NOW!
Well, "dead as a doorscrew" is probably not going to catch on.
But if no one uses doornails anymore, maybe we could come up with something new to be as dead as? After all, it's silly for doornails to only be used as things compared to which one might be said to have left this mortal coil. I grant you that you won't see a doornail get up and go dancing, but you won't see a toaster oven do that either and they're still all over the place. Nobody will be said to be as dead as a toaster oven.
So we need a new thing to be dead as. Something starting with a D, to keep that pleasant alliteration that goes so well with death. Maybe:
Dead as a dachshund
Dead as a DN100
Dead as a doody
Dead as a Denny's
Dead as a dodo
Dead as a Department of Motor Vehicles
Dead as a Datsun
Dead as a disco
Dead as a dandruff
Dead as a dik-dik
Dead as a dilophosaurus
Dead as a Doobie Brother
Dead as a Dickens
Dead as a dingleberry
Dead as Dumbledore
Dead as decorum
Mmmmmm... maybe this explains the continuing popularity of the doornail as a thing used for comparison purposes.
Say, if you were really sick, could you be said to be dead as half a doornail? Maybe if you got totally obliterated you would be dead as a whole box of doornails. There might be poetic considerations that even Dickens hasn't worked out.
Pace Dickens, I think the country will survive a little fooling around with a simile, don't you?