I wonder if the value would ever fall to precisely zero. Mightn't they still have value as a novelty or collector's item? Like, "Hey, a Bitcoin! I remember those... sure, I'll buy one, just to say I have one." It's unfortunate that you couldn't mount it in a decorative frame, though.
That's true, but as value drops, there is less incentive to mine bitcoin so there'll be a lot less mining going on(not cost-effective with respect to the cost of electricity).