Assuming that bit-coin is not passed over for a more useful digital currency at which point the network fails and the total value of all bit-coins falls to 0.
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.
At some point it would probably be easier to mine one, if the value and interest drops low enough.
That's backwards. As the value approaches zero, the relative cost of mining grows not shrinks.
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).