Re: significantly lower power degradation over time
I did a quick google, Wikipedia states that in 1977 the RTGs were generating a total of 470 watts. As you say, Pu-238 has a half-life of 88 ish years, so would lose 0.79% of their power output each year. If that reduction was consistent then today they’ would be knocking out 299ish watts.
The actual figure is lower because the thermocouples that convert heat to electricity also degrade over time, in 2011 for instance, the Voyagers were generating 270 watts, rather than 343 watts.
As of Feb this year, Voyager 1 with it's 4 working instruments only has 8.3 watts to spare. Voyager 2 with it's 5 instruments only has 3.6 watts to spare. To keep comms up NASA will have to either turn another instrument off on each, or come up with clever ideas in typical NASA fashion (along the lines of how they kept the rovers going) to somehow ration the power between all working devices, outside of the original design.
Maybe if a more efficient RTG design had been available in the 70s, the Voyagers could have been kept in service say an extra decade, before reaching the point we are at now where NASA are having to power instruments down. Hopefully in the last 47 years, we've made some progress in power conversion efficiency that can be applied to future devices.