NASA hopes to make it the 2030s with 1 remaining science instrument on each[0]. Currently, Voyager 1 has 3 remaining instruments (of 11) and Voyager 2 only has 2. ~2036 is the maximum cutoff, as then they will be out of range of the Deep Space Network[1].
[0] https://www.jpl.nasa.gov/news/nasa-turns-off-two-voyager-sci...
[1] https://science.nasa.gov/mission/voyager/frequently-asked-qu...
More and more deep space missions are orbiters or landers now, so there are fewer flyby missions that can double as interstellar medium missions like Voyager/Pioneer, but New Horizons is one of them.
I suppose an extra-solar-system probe though would simply need some gravitational slingshotting and not necessarily visit many of the outer planets. I suppose that changes the time scale.
https://youtu.be/NQFqDKRAROI?si=AzuL-NZ6JYJ71Rpj&t=883
...which might get up to 22 AU per year. And then in the future: laser-pushed light sails:
https://ia800108.us.archive.org/view_archive.php?archive=/24...
This is an unfortunate reality of our society. We've only ever spent dollars in space when it was advantageous to our Department of Defense, and the military in general.
People and companies who have succeeded in space have tied their goals to overarching military objectives. It's the best way to win the space race. Make the military understand they need to do the thing you want to do.
What of people growing up 10, 20, 30 years from now? They'll be taught in school about stuff from Voyager and then told 'and that was what we learned in the golden age of space exploration, which ended long before you were born because we couldn't be bothered to keep at it.' Having grown up in the 70s, I feel somewhat betrayed that we just just gave up on doing moon stuff, rendering a whole generation's aspirations on space exploration into a lie. The claims that 'there is nothing more to discover up/out there' is nonsense, much like the claims that 'chips can't be made any smaller' that I would hear back in the 32nm period.
The lack of long-term commitment to exploratory space is a terrible waste. To be sure we have been doing some stuff in system, but if he had kept putting out deep space probes every few years with more advanced instruments we would have learned a lot of other things by now, and we would have a long-term stream of new data coming in for the future. Now arguments for launching more deep space probes are dismissed with 'it'll take decades before we get anything useful back.' Yeah, because we stopped iterating! Meantime allowing that sort of exploration to become anachronistic is one reason we are overrun with flat-earthers and other science woo even at the highest levels of government.