The Voyager probes are powered entirely from their RTGs, they have no solar panels nor batteries[1]. So if the power output from the RTGs drops below the level needed to power the transmitter, Voyager can't talk to us anymore. There's no batteries that can be recharged and no solar panels it ever drew power from.
The exciting part from this discovery is the potential to keep talking to Voyager longer than we thought. If the S-band transmitter uses less power and we're still able to detect that signal, we may be able to communicate with the probes with lower RTG outputs than we initially thought. Though at that point, if the radio is the only thing still running and there's no instruments operating, the usefulness is probably pretty low at that point.
[1] https://en.wikipedia.org/wiki/Voyager_1#Power, https://en.wikipedia.org/wiki/Voyager_2#Power