The half-life of Pu-238 (the isotope used in these) is 87.7 years [1]. So they pretty much last a lifetime. For the same reason, Voyager 1 is still kicking even though it's out of the solar system [2].
I highly recommend this podcast episode with one of the Voyager project managers: https://www.listentospacepod.com/episodes/2016/8/5/episode-5... (about 20 minutes long, iirc)
(In fact, I'd highly recommend the whole podcast, not just this episode, but this one actually talks about power consumption and battery life of Voyager).
Just like apple limits power draw of it's devices to avoid possible reboots when too much current is demanded from an old battery.
You have to remember that Voyager is way past its originally planned mission life span, and that the nuclear battery was more than fine for the planned life span.
Batteries tend to be heavy, not operate well at very low temperatures, and wear down quicker than the 40+ years since construction of the space craft.
From the perspective of the original mission design, even with incredible foresight, it seems to be not worth the trade-offs.