I do note that the Bell history said they intended for very long
> product lifespans, so anything they made had to be able to withstand
> many years of service. I believe they didn't change this philosophy
> until the 1970s when rapidly changing electronics kept making
> components obsolete quickly.
I don't know if this is relevant to the statement you read, but regulators controlled the rate at which AT&T and the local Bell Telcos could depreciate their equipment. Longer depreciable lives meant lower expenses and reduced pressure to increase rates.
As might be expected, the carriers were always asking the regulators for faster depreciation, and the regulators always pushed back with longer lives. It wasn't until the late 80s when local regulators finally realized that an electronic or digital switch did not realistically have a 30-year life.
Fred Goodwin