The Pioneer probes leaving our solar system are slowing down are are, likely, just starting to lose their molecular cohesiveness. They are, I suspect, already on their way to becoming "radioactive" and even the simple metals will start to emit radiation as the components elements decay outside of the Sol field. The highest atomic weight elements should start emitting first and down on through the periodic chart. These emissions (once significant) in all directions would likely also slow forward momentum and the fields that these elements possess will be drawn back to our solar system - which they have a fundamental resonance with. If the probe doesn't break far enough away it will come to a complete stop and then start its return - becoming another projectile orbiting the sun, like all the asteroids.

If the probe does have enough momentum to escape the solar field, after hundreds of thousands or perhaps even millions of years, the probe could completely decay into a gaseous state - hydrogen and a few other elements supported by the interstellar galactic medium - a giant low level field - unless it makes its way into another solar system and its accompanying field. Hopefully, we can catch up with the probe and examine it with spaceships built for the interstellar medium long, long before Pioneer 10 completes its several million year trip to Aldebaran.

We need to know exactly what is going on out there. Future human exploration throughout the solar system and beyond demands nothing less than precise knowledge. Hypothesis and theory are not something you want to risk lives on.