The trajectory was a once-in-a-lifetime opportunity based on the dynamics of our solar system, and surely the relevant experts will take advantage of the next similar opportunities. It's also almost a miracle that the probes have lasted this long. They used analogue electronics and if they launched ten years later, they would have more modern electronics and would probably have failed by now. And it's not possible to simply argue to use older designs to last longer in a new mission, because you would give up multiple orders of magnitude of data collection.
It's not a simple matter of any current launcher being cheaper so we should launch a bunch more.
> And it's not possible to simply argue to use older designs to last longer in a new mission, because you would give up multiple orders of magnitude of data collection.
Why not both?
The heavy parts of the probe, then or now, were presumably not the electronics themselves so much as the physical structure, the power system, etc etc.
If you were designing a probe to last fifty, a hundred years, why not include series of fallbacks of the electronics to simpler and more basic systems, so that you could still get something out of it, after the primary state-of-the-art electronics fail?
This is like asking why a Tesla model S can't revert to a Ford model T and keep functioning for decades (without a technician allowed to touch the car).
It's not a simple matter of any current launcher being cheaper so we should launch a bunch more.