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.