So, if the debtor can afford 5% but no more, the CC company achieves their goal of clearing the bad debt for the same price they would by selling to an agency.

Why not hold out for more? The debtor has already shown an unwillingness to pay under the current terms, and time has run out for holding the bad debt.

Why would the debtor be willing/able to pay 5% but not the current terms? For a CC, it's probably the interest rate and interest balance. In this situation the CC company is usually charging 19% - 25% interest and including accumulated unpaid interest in the balance. If the debtor makes a payment, the terms say that the payment is applied to interest first, so the debtor's principal is not reduced at all until ALL of the interest is paid. That can be lifetime indenture to the CC company. A 5% payoff is a renegotiation of the terms which will terminate the account and prevent any further interest charges. That's worth scraping together the money needed for the payoff.