First, I have not offered a criticism of MMT. I have offered a criticism of Mr. Diaz's failure to mention the inflation constraint. Nothing else in any of my posts suggests that MMT is in any way incorrect.

Second, it makes no difference whether the government "must" borrow. What matters is that the government does borrow, and it pays market rates. If the Fed suppresses those rates by monetizing the debt, that just puts more inflationary money out into the economy chasing goods that aren't there. If the Fed doesn't monetize, higher rates in the private market cause an economic slow-down, i.e. the government is in effect crowding out private demand for goods by outbidding the consumer for loan funds.

The government can spend too much money, and a budgetary constraint, artificial as it may be, may prevent that from happening. OTOH, the government can also spend too little money, and a budget constraint can be the cause of that. That's why I believe Keynes would have been an MMTer if supply were more elastic in his day. As he said, when facts changed, he changed his mind. Globalization, digitization, and automation have changed the facts. Last year's actual GDP is no longer a useful measure of next year's potential GDP, and MMT explains to policymakers why they should spend into rising capacity.