As for helpdesk support... support isn't going anywhere. Although I feel like it's a fruitless pursuit to spend your entire career in.
If you're 15 years away from retirement, I would seriously be looking for opportunities for education and advancement,
to a more managerial position, where you could have more impact, and maybe get a higher inome for a better retirement.
As you mentioned.... people too far for DSL.
Aside from clear channel DS3; which I don't think is even an argument, that those are going anywhere -- businesses still buy those.
And the capacity and assurance that the bandwidth will be available is much higher than DSL.
As you didn't mention... businesses that need something more reliable than DSL, and a SLA from their telecommunications provider.
DSL is typically best-effort by the ILEC; sometimes taking 48 to 72 hours to repair. ISDN services are less fragile, and typically have a tigher SLA for diagnosis and repair -- and hey the insult required to break ISDN are essentially drastic situations like stray voltage on the line, cut or short-circuit.
DSL reception can be totally broken, or the speed suddenly greatly diminished, by a huge variety of minor insults to the copper, where electrical continuity isn't lost.
The performance you will get from a T1 link by contrast, is pretty much a certain thing, barring severe damage to the copper.
Businesses requiring POTS applications; believe it or not, VoIP doesn't work for just anything, and still might not be preferred even if it's cheaper;
the reliability and security characteristics of POTS may be preferred.
For example: IT security departments like POTS, because VoIP is so vulnerable, and easy to record, intercept, and forge calls, in case of network intrusion.
Various applications work better with POTS, such as fax machines and alarm systems.
In large sites, there is likely to be some need, and maybe enough need that a PRI or channelized T1 is required for 24 phone lines.
Existing services where T1/T3 is already in place are unlikely to be changed; where they are filling the need.
Not every business wants to tempt fate by switching kinds of service if there is no need to it --- for the forseeable future,
there is no massive exodus for DSL.
DS3 signalling isn't going anywhere either; it's the way of muxing a bunch of T1s or SLA guaranteed customer circuits for circuit protection and mapping across the transport network infrastructure. A bunch of DS0s become DS1s; a bunch of DS1s become DS3s; a bunch of DS3s become OC-xxx; a bunch of those so-called obsolete T1s form the backbone of a telco transport network.