JeffG:
"Doesn't the same logic hold true for the entire machine? How many unused 1957 Kenmores are left today?"
Up to a point, yes. It should be remembered, though that the machine has suffered some deterioration in storage, making some restoration necessary. The only way to know if restoration efforts have been entirely successful is to use the machine a bit, protecting certain parts as you do so.
Because of the deterioration that had taken place on a few rubber and plastic parts, the machine was certainly unused, but was no longer entirely mint.
What I'm trying to say here is that on balance, I think Robert is doing the machine way more good than harm. As a whole, the machine is more important and significant than it was before, because it has almost no use on it, but it is in working order. Because of Robert's custodianship of the Kennie, we not only know what those machines looked like new, we're going to know how they sounded and worked when new. We can't know that from an unused machine that is just left in storage.
Remember that with the agitator replaced, the remaining parts are ones known to hold up fine to modern detergents, like porcelain. And it's not as if Robert's going to put much mileage on this baby- I'm betting it gets only the occasional ceremonial washload (Robert, correct me if I'm wrong).
But that pristine agitator can never happen again, so I think he's smart to hold that back.