>Taking out debt at a negative interest rate has no such default risk: since you borrowed the money in the first place, if you default the lender is out the principal, not you.
I don't think this is accurate, as there is still risk because we are talking about a mortgage, not just a negative interest loan. A mortgage comes with down payment and a house for collateral. You can end up underwater on an interest free home mortgage just as easy as one with a positive mortgage. The only difference is your monthly payment is lower without mortgage interest.
Also, if you default on the mortgage, you are also out your 20% down payment.
The part I find interesting about the current situation is that stocks are sky high going higher, but loan interest is low. This indicates to me that banks and institutions are desperate to park money anywhere but the stock market despite the incredible market performance.