It is generally accepted that maintaining older equipment is more intensive than newer. Aside from parts sourcing challenges, maintenance intervals are almost always shorter on older equipment.A smaller group or type magnifies that. Sometimes, the higher cost of running an older type is balanced out by other factors. An older fully paid for plane or car may be less fuel efficient, but depending on fuel prices may still be viable if the additional maintenance and fuel cost is offset by the lower ownership cost. Other factors creep in, can a piece of equipment do the job required, in airplanes, solid planes with a 3 person crew got retired for comparable 2 person flight crew aircraft. I would be very surprised if a PPC was not quite a bit more expensive to keep on the road over time than a SSL. That doesn't mean a case can't be made to keep them alive, but if they have a minimal effect on passenger satisfaction, and don't bring in money that approaches their cost to keep around, the big question becomes, why?