Yes I was a bit puzzled too. I always thought that the 141s and 142s had steel bodies. It was mentioned on post #1061 that BR were talking about having the Pacers rebodied at some stage to extend their lives. Imagine a 142 with a shiny new Alexander Dennis or Wright body!
Apparently the 150s have bad corrosion issues too. I read about new roofs being welded onto some carriages. These too were only meant to be in service for 30 years.
In an ideal world both Pacers and 150s would be directly replaced with something like a DMU version of the Alstom X'Trapolis 100.
https://en.wikipedia.org/wiki/X'Trapolis_100