The first two are in good spots to look at stuff already, and there's probably not enough political will to send another similar mission with more modern sensor tech - especially when they took 50 years to get where they are now, and currently available rocket engines aren't much better than what was around in the '70s in terms of ability to propel something beyond the heliosphere.

It might make more sense when we've got nuclear rockets happening - some of those have mind-boggling Isp compared to chemical rockets, at least on paper!

Or perhaps if there's an upcoming favourable alignment of the interplanetary transport network or something..