Given the technology of the early 70s (very large feature size, very low level of integration, wide traces, probably not CMOS, etc) it's been possible to understand them well enough to keep them running and repurpose them. The more modern mars rovers also exceeded their mission lifespans by several times, but had ultimate lifetimes of about 15 years. And I note that one of the probes had an early failure of flash memory, something that didn't even exist in 1975. So while they exceeded their mission by more than 30X, in absolute time it was much shorter than Voyager.
But outside that kind of almost unlimited budget environment are we still building things with unanticipated lifespans? Everything from electronics to housing is optimized for cost of manufacture, not long term operating value. And that extends far enough through the stack (from semiconductor to UI) that I wonder if design for the long term is even possible.
Sure, a few hackers can reflash an old phone to be a TV remote, but the bulk of the population can't or can't be bothered. More dangerously, in a few years people will be scattering IoT devices willy-nilly in fields, over provisioning because it's OK if a few fail if you have enough running. When they stop working they'll just drop more, newer ones in; it won't be worth picking up the old ones which will just become e-waste in situ.
Is it even reasonable to want active devices to be designed with long lifetimes? Outside space programs do we even have the skills and technologies?
If it were built by the same rote as every other probe with short term life and cost as a high priority, probably not.