In the case of bitcoin should the public don't use it anymore, the price would collapse, making the current mining unsustainable, and as the mining difficulty declines the robustness of the protocol reduces sharply, especially if state actors start buying the unused mining devices…
BTW, the sneakernet is unsuited for most uses of bitcoin transactions: you don't just want to be able to transfer the token from one address to another. Most of the time a transaction is an interactive process: you send the money and I give you something in return after I've checked that I actually received the funds. This usage simply doesn't work well on the sneakernet, because of the very high latency.