They also have better, more ubiquitous technology. Is an off-the-shelf managed Ethernet switch on a UPS really less reliable per user than a 5ESS? I don't know, but surely only marginally because both are pretty close to 100%, and the main cause of failures is human errors on both systems.

The actual problem people have with VoIP landlines is that they require power at the customer end. A traditional analog POTS line supplies power from the central office so it doesn't need any from the customer, unless they want to use a cordless phone, which takes more power than the central office provides. A fiber VoIP line requires power at both ends no matter what.

Note the original phones actually didn't have "central battery" as they call it. Instead each phone had its own battery, designed for about 6 months of usage, and every 6 months the phone company sent a wagon around to replace everyone's phone batteries.