Re: There's only one reason to tear out copper
"with FTTP VoIP your reliant on the 8-16 hour UPS to keep the Fiber working for the phone"
I would think that the time you need to use the phone after a power failure is immediately after the event. The backup battery is intended to handle temporary outages of the old "line fell" or "rat chewed the wires": maybe worse-case a blown transformer. I know one of the first things I do is call the power company to either report the problem or learn how long it'll probably take to get back on. If I'm going to call anyone, it's going to be right after. Beyond this and you're probably going into disaster territory. Based on experience (I live in a hurricane zone), I can say that if your power isn't back on (or due back) within 12 hours, there are other problems. If you can't even REACH the power company (because the phone is cut off upstream), you might as well hunker down; you're in for a wait, in which case the phone isn't going to be of much use to you anyway. In any event, if an emergency arises during such a disaster (like a flooding that can warrant evacuation), the order will likely come from a truck in the street rather than the phone.
"some Primarily mobile masts have a backup generator or backup batteries"
SOME, but not many. So the cell phone isn't likely to be of much help in a major outage, either, unless you're willing to roam to find a working tower.