The demand for DAI comes people who want to repay their DAI denomonated debt to unlock their collateral or stave off liquidation, or from liquidators who need DAI to participate in a liquidation auction that happens if a vault holder's collateral falls below a target threshold. In any of those cases, when the DAI is repaid, it gets burned. As long as folks want their collateral back, or liquidators want to buy that collateral, there is demand for DAI.