When you're far enough from the sun to avoid thermal cycles, in the vacuum of space, what forces will cause things to deteriorate? If I wanted something to last forever, deep space would be the perfect place for it.
-- Many materials age on their own; plasticizers outgas from plastics. You're probably familiar with how once-pliable but now old/aged plastics, even kept sealed in a box, can become crunchy and brittle.
-- Chemistry doesn't stop in space. The thrusters are likely to involve chemically-reactive materials. Any little bit of corrosion, stress-corrosion cracking, etcetera could cause anomalous performance.
It is wonderful when things still work.
See e.g. this study on plastics erosion in LEO: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...
Not sure how it varies as you leave the solar system - depends on the exact balance between Solar Energetic Particles (particle radiation from the sun) and Cosmic Rays (particle radiation from outside the solar system), and on how much the cosmic radiation increases as Voyager leaves the heliosphere.