What if there was a bigger fish that could come in and offset their asset imbalance?
The high levels of private debt are not the problem, they're a manifestation of the real problem which is that successive governments have starved the real economy of funds needed to grow. The level of private-sector debt is equal to the shortfall in government spending in the real economy.
A debt jubilee will only make the problem of precariously high debt levels go away for a short time, it won't solve what caused the economy to become so dependent upon private debt in the first place ie wages being squeezed and a reluctance among the business sector to expand. And then it wouldn't be long in our new debt-free world before we'd be relying on rising levels of private debt to fuel economic growth again.