The divine debtor theory of atonement is a variant on penal substitution without the horror of a vengeful god getting out his aggression by beating up Jesus.
The human race, though its renunciation of an aboriginal and necessary gift, is a failed project that now dies out in justice. Christ dies instead of human beings, not because human beings need not die, but because they no longer die to satisfy justice.*
Here comes the metaphor: if you owe me a fine and someone else pays it for you, then your giving me money is no longer something owed in justice but something that makes me your debtor.
Here’s the cash value of the metaphor: If death is no longer a punishment (i.e. a sort of good) then it is an evil that is incompatible with God’s existence and so must ultimately be done away with (the argument from evil is sound but simply fails to appreciate that the evils of the universe must be justified when its whole story is told). If the penalty has been paid then my death is an injustice for which God must make some ultimate remedy. The death of Christ is an act done in mercy that allows for the resurrection of the human race in justice.
*Except in the the sense that all unrepented actual sin can require death as a matter of justice. Those dying in such a state have special, extrinsic circumstances excluding them from the intrinsically universal scope of the redemptive act.