The discussion about central bank digital currencies (CBDC) has gained an impressive momentum. So far, however, the main focus has been on the macroeconomic implications of CBDCs and the narrow perspective of developing a digital substitute for cash. This paper adds a microeconomic dimension of CBDC to the discussion. We provide an overview of the existing payment ecosystem and derive a systemic taxonomy of CBDCs that distinguishes between new payment objects and new payment systems. Using our systemic taxonomy, we are able to categorize different CBDC proposals. In order to discuss and evaluate the different CBDC design options, we develop two criteria: allocative efficiency, i.e. whether a market failure can be diagnosed that justifies a government intervention, and attractiveness for users, i.e. whether CBDC proposals constitute attractive alternatives for users compared to existing payment objects and payment systems. Our analysis shows that there is no justification for digital cash substitutes from the point of view of allocative efficiency and the user perspective. Instead, our analysis opens the perspective for a retail payment system organized or orchestrated by the central bank without a new, independent payment object.