What is the most likely etymology of "dead as a doornail"

So it turns out that a doornail isn't just a nail in a door, but a nail that cannot be removed and reused. Way back when, nails were made by hand and quite valuable. People would salvage and repurpose nails whenever they could. The way doornails were bent and driven into the backside of a door made it virtually impossible for them to be reused as a nail.

So not only are doornails dead simply because they're nails, but because their future potential for any other use is also dead. They are doubly dead, if you will. Extra deceased.

One plausible explanation is that doors were built using only wood boards and hand-forged nails: the nails were long enough to dead nail the (vertical) wooden panels and (horizontal) stretcher boards securely together, so they would not easily pull apart. This was done by pounding the protruding point of the nail over and down into the wood. A nail that was bent in this fashion (and thus not easily pulled out) was said to be "dead", thus dead as a doornail.