Well, technically Pioneer 10 & 11 and Voyager 1 & 2 could be considered interstellar probes, since they're all on trajectories out of the Solar System, though none of them has verifiably crossed the heliosheath into interstellar space as yet. And the New Horizons probe to Pluto is scheduled to leave the Solar System in 2029.
But yeah, aside from that, point taken. The most likely discoveries that would trigger the development of an intentionally interstellar mission would be:
1) The discovery of an Earthlike planet around one or more stars of Alpha Centauri.
2) The discovery of one or more brown dwarfs closer to us than Alpha Centauri is.
Both of these have a reasonably high probability of happening, probably within the next few years.