...the Voyager RTG cores are still putting out plenty of heat. The problem is that the thermoelectric converters that turn that heat into electricity have worn out.
The isotope's decay rate is fixed by physics, 238Pu has a half-life of 87.7 years, which means a 1/e life of about 119.1 years or a year-on-year decay of about 0.8%. (envelope-back checking my math; (1-0.008)87.7 is indeed roughly 0.5)
Do the thermocouples on the older RTGs (circa Voyagers) actually lose efficiency at a faster rate than the 238Pu loses itself? Is it the heat or the radiation that gets them? Are modern RTG thermocouples longer-lived?