[azernik]

Voyagers 1 and 2 took advantage of a fleeting phenomenon - an alignment of all the outer planets that allowed successive gravity assists, and so let us send probes out to those planets and then on to escape velocity with a very small launcher/budget. These come around every 175 years, so the next point at which solar system escape will be that easy is around 2150.

https://en.wikipedia.org/wiki/Grand_Tour_program

All the ideas thrown around for follow-on missions have relied either on very powerful boosters (on the scale of SLS), smaller probes (New Horizons, a bit over half the size of the Voyagers and on a much lower-energy trajectory, despite a more powerful launcher), or an Oberth maneuver close to the sun in a very challenging thermal and radiation environment.