Could not the dying Old Mr. Darcy have assured the still healthy Old Mr. Wickham that his son would be provided for?

"My excellent father died about five years ago; and his attachment to Mr. Wickham was to the last so steady, that in his will he particularly recommended it to me, to promote his advancement in the best manner that his profession might allow.... (Ch. 35)

Old Mr. Darcy may simply have reiterated this before he died.

"Yes -- the late Mr. Darcy bequeathed me the next presentation of the best living in his gift." ... "There was just such an informality in the terms of the bequest as to give me no hope from law. ..." (Ch. 16)