The mechanical aspect of this has nothing whatsoever to do with why it wouldn't last for millennia - it's the fact that it's coupled to a resistive load that will dissipate power from the battery much faster than the motor itself uses, and act to brake it over time as the battery voltage drops.
The electronics and heaters aboard each nearly one-ton Voyager spacecraft can operate on only 400 watts of power, or roughly one-fourth that used by an average residential home in the western United States.
A set of small thrusters provides Voyager with the capability for attitude control and trajectory correction. Each of these tiny assemblies has a thrust of only three ounces. In the absence of friction, on a level road, it would take nearly six hours to accelerate a large car up to a speed of 48 km/h (30 mph) using one of the thrusters.
The Voyager scan platform can be moved about two axes of rotation. A thumb-sized motor in the gear train drive assembly (which turns 9000 revolutions for each single revolution of the scan platform) will have rotated five million revolutions from launch through the Neptune encounter. This is equivalent to the number of automobile crankshaft revolutions during a trip of 2725 km (1700 mi), about the distance from Boston,MA to Dallas,TX.
The Voyager gyroscopes can detect spacecraft angular motion as little as one ten-thousandth of a degree.
JPL knows. BUt more importantly, when the original thrusters broke after 37 years, they were able to switch the backups and it worked fine. To me, remotely fixing a space probe beats a spinning magnetic bearing on earth for amazing engineering any day.
