Why a high debt-to-GDP ratio makes the economy dangerous

Why a high debt-to-GDP ratio makes the economy dangerous

The reason why debt is interesting is that no matter how much debt increases, the net debt of society as a whole is zero because bonds and debts occur at the same time for one debt.
(However, in the case of the IMF, the net debt of society also increased because it borrowed money from outside society.)
If it's just domestic debt, no matter how much debt increases, it doesn't have much impact on the economy because net debt is zero...
I don't think so.
Because there are cases of non-repayment
Non-repayment occurs when the following two overlap
a decline in the value of collateral
And
In the case where income cannot afford interest or repayment principal and interest.

Even if net debt is zero, the greater the total amount of debt, the greater the likelihood of non-repayment
Let's take a comparison between a lease and a mortgage, for example
1)Mortgage loans
Landlord A borrows 500 million from the bank as collateral for a billion-dollar house
2)The lease on a deposit basis is
Tenant B got a 500 million lease loan from the bank
Again, it lends 500 million won to landlord A

1)In this case, net debt is zero and total debt is 500 million
2)In the case of, net debt is zero and total debt is one billion
1)In this case, we look at whether the collateral value of the house is maintained and whether the income of landlord A is maintained enough to repay the principal and interest
2)In this case, it is necessary to see whether landlord A's repayment capacity is maintained in addition to the collateral value of the house and tenant B's repayment capacity
In other words, even if the net debt remains the same, it can be seen that the higher the total debt ratio, the higher the uncertainty of repayment

And, in modern society, the ability to repay debts is often linked by chains, so there is a high possibility that the inability to repay on one side of society will spread throughout society
2)As in the case of a lease on a deposit basis, if the landlord fails to pay the lease on a deposit basis, this leads to the tenant's inability to repay it