This is forecast to happen near the year 2015, which may or may not be after they have crossed the heliopause. Electrical energy from their RTGs may fall below a useable level about the same time or shortly thereafter. The spacecraft's supply of hydrazine may become depleted sometime after that, making continued three-axis stabilization impossible. Pioneer 12 ran out of hydrazine propellant in 1993, and was unable to further resist the slow decay of its orbit resulting from friction with the tenuous upper atmosphere of Venus. It entered the atmosphere and burned up like a meteor after fourteen years of service.
Components wear out and fail.
The Hubble Space Telescope has been fitted with many new components, including new attitude-reference gyroscopes, to replace failed and failing units. Two of Magellan's attitude-reference gyroscopes had failed prior to the start of the transition experiment, but of course no replacement was possible. To date, a JPL mission has not been turned off because of lack of funding. Once a mission has ended, the flight team personnel are disbanded, and the ground hardware is returned to the loan pool or sent into long-term storage. Sometimes it is possible to donate excess computers to schools.