The coax networks that GP refers to have nothing to do with cable television (except that television syndication links were carried over them). GP is referring to the AT&T L-series carriers which carried telephone calls (and later digital voice link equivalents) over multiple bundled coaxial pairs.

The L-carrier infrastructure was built to remarkable standards of durability and reliability, in part because much of the system (particularly L-3I and L-4) were built as part of federal C3I contracts and were hardened against limited nuclear strikes, but also because of AT&T/WECo's general culture of obsession with reliability. Station equipment would be entirely duplicated for redundancy, nearly all equipment stations were manned to allow for quick repair (and 'maintenance patrols' inspected remote en-route repeaters as often as daily), and the system was designed for network-level redundancy through rerouting and the preparation of 'alternate control points' to allow continued operation even in the case of complete isolation of parts of the network.

The L-carrier system fell out of use when it was replaced by fiber-optic technology (which AT&T initially referred to as "lightguide", perhaps due to it coming hot on the tails of their work on waveguide carriers such as WT4 which were essentially microwave radio through a metal duct).

Of course, this raises the question of why this infrastructure is so much more reliable than cable television seems to be. I would suggest three explanations. First, AT&T, Bell Labs, WECo, and the rest of the Ma Bell crowd held reliability as an extremely high value, at both a cultural and requirements level. This is exemplified by now legendary Bell system feats such as moving exchange office buildings with operators still working inside (and dragging long tether cables) in order to avoid a service interruption. Second, AT&T long held major defense contracts with significant reliability requirements (against both natural conditions and enemy attack), and as a matter of cost savings these contracts were met using infrastructure shared with civilian services. In general, the non-Bell system modern internet carriers have not held this type of business on such a large scale, in part because they postdate the cold war and in part because AT&T, CenturyLink, and other Bell alums are more interested in federal contracts.

Third, cable television has historically been both more competitive and less critical in popular opinion than telephone service, and so reliability has simply not been a high business priority for these operators. For the Bell system any service interruption could be a major scandal, and this is still to some extent true today of telephone service (see any 911 interruption incident). On the other hand the cable companies descended from offering a service which was viewed as a non-critical luxury. Consumer and regulator behavior seems to have just not caught up to the internet era in this regard.

And yes, it is quite true that many of the Bell system alums are struggling today. On the other hand, CenturyLink is quite viable as a business and shows few signs of slowing. They are even arguably relatively innovation-forward as an ISP as evidenced by their having one of the larger gigabit GPON footprints in the US, as well as emphasizing no-contract "price for life" service in many markets where they compete with cable carriers. But yes, ultimately most of the Bell system has been dismantled in the interest of cost savings, and has been replaced by a sort of shambling wisp of its former self. Many feel that the writing was on the wall as soon as the breakup occurred, a move which was in good part aimed at reducing consumer prices, and did so, but partially at the cost of radically reduced investment in infrastructure. There is tradeoff between affordability and reliability, and the telecom industry seems to always cling to one of two extremes.