Contracts don't exist within currencies; contracts exist on top of currencies. Sticking a turing complete language into the currency doesn't remove the problem

Contracts don't exist within currencies; contracts exist on top of currencies. Sticking a turing complete language into the currency doesn't remove the problem of third party management; it forces the currency to absorb some of the complexity. Is there any reason why this can't just be layered over an existing crypto currency instead of needing to be built in?