If a VanishingCoin will lose half its value in a week, and I buy and sell it in a second, using it as a fast cheap safe transaction, then my cost will be at least equal to (1/2)^(1 sec/1 week) of the transaction. VanishingCoin will be outcompeted by SteadyCoin, which is just as fast, cheap and safe, and doesn't lose its value.