Future returns to the DAO don't necessarily need to be held in ETH, so the capitalization argument fails. Imagine (unrealistically of course) DAO makes a speculator investment that pays a daily return at an annualized rate of 50% forever. These returns may be passed through as received to the DAO members who then, we shall further assume, dump then as received. The value of DAO tokens would then increase enormously but the value of ETH would not. In theory the market cap of DAO tokens could exceed the market cap of ETH. This does not make the DAO a good investment, of course.