Unlike Hubble, Webb is not designed to be serviced. Webb will be located at a much greater distance
than Hubble, beyond the Moon instead of orbiting just above Earth. This is the reason that the nominal science mission length is 5 years with a 10-year goal. To insure the 5-year mission, NASA has engineered the observatory so that all critical subsystems are dual or will degrade gracefully with age. For instance, the Near Infrared Camera has two identical camera systems so that the optical quality can be maintained even if one were to fail.
Webb will also contain enough fuel for 10 years of maneuvers at its location. As with Hubble, Chandra, and Spitzer, the Webb science and operations center has the ability to change the operations of the observatory to maximize its scientific potential as it ages.