He's not just arguing about the value of the degree itself -- he's arguing that the value of the degree, coupled with the rising costs (which are increasing at roughly double the rate of inflation), make the long-term value proposition of a college degree far less certain.

Furthermore, I think his argument is less focused on the value of the degree itself, and more on the value of subsidies for college tuition. Just as cheap lending made homes more expensive, cheap lending can/will/does make tuition more expensive. Is subsidizing a college education for everyone such a good idea, if it leads to enormous costs?

I don't know the answer, but I think that the argument is more complex than "degrees aren't worth as much when everyone has them."