Even from the other side of the globe I can tell that it seems strangely short-lived. Built for a decade, then used for a decade?
Wikipedia notes tersely:
> The launch of communication satellites in the 1970s provided a cheaper alternative.
In the early 1930s, both time and frequency multiplexing were introduced on coaxial cable, carrying up to 600 channels per conductor, it rendered the above twisted pair multiplexed lines obsolete within about a decade.
c. 1950 microwave relay is introduced (the AT&T long lines in question here) and undergoes explosive growth, with the cross-continental system linking most major cities built by 1960, and reaching essentially every part of the country by 1970. Such links could carry hundreds of megabit/s worth of data (or multiple 6 MHz TV analogue channels). Much cheaper than coaxial, it largely halted new coax installations. It would remain dominant into the 1980s, so more like 20 years.
Fibre optic communication was introduced in the mid-1970s. Immune to atmospheric problems, less path loss, potentially capable of carrying far more data than microwave. It made most of the microwave systems obsolete by the 1990s.
Microwave relays were obsoleted by fiber optics.
https://www.telcomhistory.org/resources/online-exhibits/scie...
e.g. I found this government report report from 1983 stating that "microwave is the chief means of transmitting long-distance telephone calls..." in the U.S., this is well after the advent of satellites: https://www.google.com/books/edition/A_Five_year_plan/NbGS1A...
They effectively had a commercial life of 35-40 years, and went thru 2-3 generations of technology, some of them were even upgraded to digital.