The electric motors that drive the scan mirrors in the Imager and Sounder have double-redundant windings. Each winding can deliver enough torque to move the mirrors.
However, the windings are encased in a glassy potting compound with about 10 times the thermal coefficient of expansion of copper. So, when heated/cooled, the potting swells and shrinks and cracks, pulling apart the wires within the cracks and eventually breaking them.
One set of windings has broken in the GOES-8 Sounder and one set in the GOES-9 Imager. When the other set breaks, the mirrors will not be moveable.
When? No one can say, since "crackology" comes down to chance location. In ground testing of spare motors, 2 out of 11 motors with similar single-winding failures have failed completely. Motors with one broken winding have gone a hundred to a thousand more thermal cycles before breaking the second winding. So, you might guess 1 to 3 years life remaining on the GOES-8 Sounder and GOES-9 Imager motors, plus-or-minus a few years.
Meanwhile, the windings in the GOES-K (and after) instruments have been replaced with motors that do not have this flaw.