Passenger carrying airlines have to consider the price of fuel, the cost of cabin re-fit and maintenance checks and public perceptions/marketing issues in the when-to-retire decisions they make.
On the cargo group side:
If the airframe has some cycles left in it, an old widebody is a money making proposition for freight haulers. Extra points scored for a frame previously operated and maintained by one of the majors. These can come with an inventory of good but cheap spares too.
By the way, your list of moldy oldies is incomplete without the A300/A310 series. There are quite a few of these old gals still lumbering about the skies full of cardboard boxes and what-not.