…an approximately expect fee proceeds equivalent to some percentage of the DAI market cap each year. Given a $10B DAI market cap and a 0.5% return to MKR holders, MKR holders should expect to indirectly receive $50M through MKR burning. Note that MKR holders are also punished through dilution if CDPs become undercollateralized, which has not happened yet to my knowledge but could in a severe market crash. It might be appropriate to model this risk as a reduction to the effective interest rate.
MKR holders are not the only ones who are punished. DAI holders similarly can be forced to receive a haircut — see:
ETH price crash
The greatest threat to Single-Collateral Dai will of course always be a massive crash in price of ETH, since ETH is the only collateral type. If the price of ETH crashes so much that the system becomes net undercollateralized, all PETH and CDPs become worthless. Importantly, Dai holders will keep their claim on the leftover ETH. When global settlement is activated after a massive price crash, Dai holders will all receive a proportional claim to the underlying ETH, just like is normally the case with Global Settlement. However, Dai holders may also suffer a haircut in this scenario if the total value of the underlying ETH isn’t sufficient to cover the face value of the total outstanding Dai. ETH can’t be siphoned off by third parties or unfairly claimed by participants in this scenario.
As an example, if the total supply of Dai is 1 million, and the total value of the underlying ETH collateral is 800,000 USD, and global settlement is activated, every Dai will become a claim on 0.8 USD of ETH (based on the ETH/USD price feed at the time of global settlement). This would mean a 20% haircut to Dai holders.