For real. Most likely those planes were retired for their inefficiency.

Probably not just for inefficiency. There are many expensive wearable parts whose life are measured in hours of operation for flightworthiness. Once these reach the end of their certified life they have to be replaced before being certified for flight again (periodic engine overhaul). I could see a bunch of these be still running engines, but the cost of a certified overhaul to carry people is too expensive. However a stationary engine attached to a generator, in a farm of generators, breaking down is an annoyance, not nationwide news and a lawsuit from the families of the survivors.