Everyone would take huge loans and bet on the stock market knowing they never needed to pay them off. It would need to be announced and executed at the same time.
In 1970s you could have taken out fixed-interest rate debt (if you had the foresight), which would have been devalued as part of the 1970s inflation due to the 1971 Nixon Shock, so perhaps if you knew that you could come out ahead at the time.
Usually debt jubilees work by transferring private debts onto public sector and then having the public sector default in real terms (by devaluing the currency).