However, just because we are inclined to concupiscence, doesn't necessarily mean we lack free will entirely. Augustine believes we still have the ability to make our own choices: liberium arbitrium (the ability to make choices free from predestination). Concupiscence overrides this insofar as we are unable to make choices that lead to salvation.
Pelagius, however, objects that punishment would be unjust if predestination were true: if we can't help our sinful nature, then surely it would be unfair to have punishment inflicted on us for it? This is not a reasonable view of moral responsibility or of god. Pelagius therefore argues that we must possess free will, and sin is a free choice - hence god's justice and punishment seems reasonable.
Augustine's response to Pelagius is that our punishment for sin is deserved - but not through individual guilt, but inherited guilt through Adam. This doesn't sit well with modern views of individual personal responsibility; even Augustine seems uncomfortable about it and describes it as "secret yet just judgement of god". He quotes Psalm 25:10: ‘All the paths of the Lord are mercy and truth,’ and concludes: "neither can his grace be unjust, nor his justice cruel”.
Overall, Augustine's view is indefensible, precisely on the grounds raised by Pelagius: either god's omnibenevolence is undermined because his punishment seems cruel, or his his justice seems simply arbitrary. Punishment for what we are not ourselves responsible for seems unjust.
OR: Overall, Augustine's view, though challenging, is defensible: an overly individualistic view of personal guilt and punishment is very specific to modern western society, and the idea of collective guilt and justice is something we are coming to appreciate more and more with the recognition of things like systemic racism and sexism that we are complicit in regardless of individual choices. Augustine's view of punishment and guilt rightly views all human behaviour in communal as well as individual terms. Moreover, at some point any theist needs to recognise the mystery of god, and Augustine's view of human nature both recognises our actual experience of the world and leaves space for god's mystery.