Eventually, its power source will die out, its components will fail, or enough cosmic rays or debris will corrupt enough of its systems it can no longer transmit.
Given those factors, I do wonder what the expected lifetime of the probe is, with the power source being the most clear "game over" line.
It appears that both Voyagers are slated to have some experiments disabled soon (2020 and 2021) to best use remaining power from the RTGs, but since the RTGs are nuclear sources, the decay rate is independent of power draw (it's more an issue of switching off the experiments to avoid peak draw killing the ship).
Voy1's expected to lose too much wattage to turn on the radio by 2025; I can't find a quick number for Voy2.
EDIT: It appears Voy2's power package was expected to allow mission operation through 2020, so sayeth Wikipedia; no idea if that implies the radio was expected to die this year or this was the year the craft would need to be reconfigured for a scaled-back mission profile to maintain enough power for attitude control and radio transmission.