Like someone else commented, the poster uses terms "Copper" and "ISDN" interchangeably. However, with the inclusion of terms like T1/T3, it's clearly about "what can an old telco-guy do in this newfangled IP-based world with 15 years before retirement". Copper here is a misnomer, a lot of stuff can happen over copper (DSLs being the most obvious example).
I have some familiarity in just how dead the technology is. We have a big customer who just placed a big order for Cisco's PVDM digital modems. Why "big", if the tech is dying? Well, that stuff is going to end-of-sale after this summer and they have lot of legacy systems around the globe that dial in (machine-to-machine stuff, and not easily upgradeable everywhere at once). They are moving to IP-based systems but cannot really do that fast enough. Anyway, one of the biggest vendors of network equipment just decided that they aren't going to sell modems that can talk directly to E1/T1 line (analog 2-port models are still in the selection though). I don't know that anyone else is selling such stuff either (Alcatel maybe?). That technology had it's day, but it's long gone.
There might of course be places where, due to signaling constraints, you need to run a E1/T1, but it doesn't really use any of the features. You just run PPP over that link and be done with it - no one cares about the intricasies of Q.931 framing or setting up calls for such links. Even in telephony, it will continue to have some uses, for example many PBX systems still only provide E1/T1 uplink - even if it's going to be used just to connect couple of feet to the SIP gateway right at the next rack.
Frankly, your father has two choices: Either
a) Get entrenched into some niche that really can keep on going with ISDN-based technologies for the next 15 years - you know, maintain job security by being the "only one left who understands this piece of legacy junk that we cannot migrate away from fast". Frankly, I find such positions hard to imagine - sure, maybe if he was retiring in this decade, it could work, but hardly in the 2020's.
b) Join the IP world. Frankly, I would think that with a reasonable effort he could still become an expert in VoIP - you still need skills like provisioning (for QoS), codecs (even the G.711a/mu-law is relevant), and so on. Lot of the concepts in SIP are still based on the good old stuff from telco days. You just need to wrap your head around the concept that instead of TDM sending each frame at exactly right intervals, you get packets that might occasionally get lost or routed wrongly or arrive out-of-order...And frankly, you also don't need to care anymore about stuff like SPID's or TEIs. Which I would think of a relief.