I don't think this should have been down voted.
The "answer" is as much about human nature rather than the nature of the designers of this particular mission.
- There are often loosely declared motives for doing a government mission in the first place, though given they had decided to go ahead there comes a point very early on in a project's conception where there has to be a specific reason for everything on a space mission, the more so the more expensive it is.
- Humans, as a whole, don't have a great track record for looking far ahead. People who do that tend to be dismissed for going against the grain, unless they have lots of money.
- It was launched ~48 years ago, that is a long time for any space mission.
Speculation:
I doubt the notion of the mission being extended was that prominent in their minds during design in the early 70s.
There probably wouldn't have been much confidence that the satellite would still be operating by that time. Its possible the design was to provide enough power for the foreseeable life of the vehicle.
You can call it short sighted, but relative to what? Not so much compared to the rest of humanity.
They could have done it differently but that takes us back to the first point about motives for a space mission. There is a structural issue about how projects are organised. The designers who sorted out these problems were not in the room when their bosses' bosses' bosses committed to the project, they probably never even met them.