I feel Darcy is both pleased that Elizabeth's response suggests she has no interest in defending Wickham, or even continuing the subject, and thus also suggesting she no longer had any interest in Wickham. I personally think Darcy would have been thinking of Georgiana and the elopement as well at this point, but only in the sense of saving her feelings and ending as quickly as possible a subject that must be very painful to his sister. IMO, the use of the word 'interest' in the context of Georgiana's 'interest in the affair' suggests that while the 'affair' of her near elopement was brought to mind, he did not see his sister as accountable or to blame for being persuaded by a scoundrel when she was so young and probably naive.