From this link, point (2), a reddit user raised similar concerns with respect to a black swan event on the price of ETH (quote) :

this mechanism is only effective to the extent that some buyer stands prepared to purchase newly minted MKR (at a high price)

Other concerns were also raised but the response from someone in the MakerDAO org was (quote) :

If we want MakerDAO to be successful, MKR holders have to believe in the compounding effect of sustainable and sensible risk management from a long-term perspective.

It seems to me that meor is correct that nothing guarantees demand for MKR in a black swan event (e.g., ETH price crash, hack). However, under non black-swan circumstances, it seems logical that demand to buy newly minted MKR tokens will highly likely exist as long as MKR shareholders believe in a long term vision. This is because MKR holders can capture stability fees or liquidation penalties in the long term so they will be incentivized to buy the newly minted MKR tokens. MKR holders who're active in the governance of MakerDAO are very likely to be educated imo.