Mathematically, that's a pretty strong argument, since Bitcoin might be worth something like. $0.000001 USD which is still not nothing.
I think people would pay ~10 USD give or take an order of magnitude, for such a curiosity.
One could think to that without the BTC network, and just run an exchange of the private keys. But if you just "sell" a the private key, couldn't you just keep a copy? Wouldn't you be able to do a double spend, and sell the keys to different people? So, the only way to prove that you own the coins is by moving them a wallet that only you control. That implies that the network still needs to be alive and secure.
In short, if the BTC network collapses, the price will go to zero. There is no way around it. There is no "I'm willing to pay X for one".