None of what you have just said justifies JW having played his last game for the club. He may go at the end of the season but he certainly hasn't played his last game for the club. He will be very useful for the rest of the season and I would personally much rather see him starting at centre back than Ferdinand.
He might turn his ankle in training tomorrow or Chrissy Powell might straight away stumble upon the right back 4 who are then ever present for the remainder of the season, but otherwise I agree with you.