Letting the debtors unpunished is like letting your kid unpunished after he repeatedly plays with matches - the'll burn your house down. The fact that we spend money to run prisons is not a valid argument, that only has to do with prisons and how they manage their work.
Without such law, we'll have even more debt, as people won't have enough motivation to find ways to repay what they owe.
People should not be jailed for being in debt. There are avenues to collect money that is due and those should be exercised. Almost every student in America today will begin their professional degree so far in debt that they will never realistically pay it off in their lifetime. Interest rates should be lowered or eliminated and people should have the opportunity to pay what they owe. Jailing everyone who can't afford their payments will put undue strain on the legal system and by extension the taxpaying citizens who still have the opportunity to support themselves.
Mass incarceration has a huge cost on society; both in terms of the monetary support needed to run prisons and the lives that are impacted by serving time in prison. Locking up people who cannot pay their debts would only add to the costs of incarcerating so many people. Instead, we should reform our bankruptcy laws in a way that still hold people responsible for the money they owe, but does not ruin them for the rest of their lives either. For example, maybe some poor people could pay off some of their debts which would be better than not paying back anything at all.
If people are locked up for being poor, we are restricting their ability to get a job and try to pay off their debts. There are less harsh penalties they could be given that could encourage them to pay off their debts and to try to get out of debt. Sitting in jail solves nothing for them.
Debtors prison is antiquated and almost seems medieval. There is no reason to lock people up in prison and duhumanize them just because they can't pay their bills on time. Corporations get away with a lot of misdeeds and I think they need to have some leniency in the laws for everyday people.