It's the other way round: POTS is still the most reliable thing we have. The "intelligent" services in call and dispatching centers are what fail.
The more the telephony services are mingled with Internet, the more they are exposed to both Internet security threats and bungle-ups created when applying the various patches and updates that are made necessarily by the constant stream of security updates. Plain POTS has less of that.
It's true that if you put enough quality and maintenance into a system, it can stay at ~100% uptime. But very few organizations have those kinds of resources, and government systems tend to not be funded more than they need to be, and generally aren't staffed with state of the art tech teams.
But it's not unreasonable to imagine the possibility that this can change in the future, if the cost and risk for maintaining such a system were to decrease below a certain threshold.
Redundancy cost money, and most companies do not want to pay for it.
I work for a company with about 40 employees. Our offices are connected to two different ISPs: one by optical fiber and one by VDSL. We have two different telephone operators for our outgoing calls from the offices, and I have two mobile phones with different operators.
During the last five years there have been multiple outages that lasted between minutes and days, but most of the times the redundancy have saved us. But when we lost power (the whole city went dark for hours) we went down when the batteries in the UPS was drained. Having our own generators is too expensive.
We can, we should and in some countries we do, but the increasing reliance of POTS services on internet technology make them vulnerable to the same weaknesses that exist in the internet.
Not in this case, IMS Core was offline for the whole country which is the backing system behind pretty much all fixed infrastructure for POTS, ISDN and VoIP ingress/egress as well.
Well, IMS stands for "IP Multimedia Subsystem" and having IMS in the telephone network is, in my view, basically a part of the process of moving POTS services to IP and Internet technologies... of course, that is necessary for many of the centralized services that phone users are used to, but it is a possible point of failure of epic proportions.
The more the telephony services are mingled with Internet, the more they are exposed to both Internet security threats and bungle-ups created when applying the various patches and updates that are made necessarily by the constant stream of security updates. Plain POTS has less of that.