This is a pretty emotional subject to many, but we too are of the opinion that we should dress nicely for dinner.
Even on the last night of our cruises, my husband typical still wears a sport coat over an open collar shirt and I wear nice slacks and blouse. If it is going to be a really long flight home and we feel we need to be more comfortable, and since luggage pick up is not until 11:00pm or so, we will usually go back to our room after dinner and stuff our dinner clothes into the luggage and put on our traveling clothes for the next day - then head to bar for a nightcap. I have to travel in special tennis shoes due to arthritis when we do a lot of walking (like changing terminals!), but there is no way I would wear tennis shoes to the dining room, even if it is the last night of the cruise!