XCP Used up in smart contracts

I am new to counterparty. I have read a lot about it, I like the concept in general. I just have one problem with it. If XCP is used up like “gas” to power the smart contracts and the total amount of XCP is deteriorating as it is used up by the system running contracts etc, especially if couterparty was to used more, like Ethereum is. What happens when the circulating supply of XCP reduces to zero. ???