Quite often Christmas Eve flights, like New Year's Eve flights, do not totally sell out. 80 shy on an L-1011, in Air Transat seating, means that the plane's seats were all filled up, but there was still some room in the overhead bins, the aisles, and the lavatories.
Those planes may be old, but they are still perfectly airworthy. And they are paid for. That's why they keep them. They may burn more gas, and cost more to maintain, but considering they are, otherwise, free, they still end up being a sensible solution for a low-fare charter company like Air Transat.
On a similar note, the USAF still flies the B-52 bomber 50 years after the model entered service. The youngest B-52 in the fleet came off the assembly line in 1962. Yet, amazingly, the USAF plans to keep those newer B-52s flying for an additional 38 years. They do not plan on retiring them until the year 2041. By that time, it would not shock me to learn that the B-52 was to be replaced with the L-1011!
An optimist robs himself of the joy of being pleasantly surprised