@Heopps' cool answer to the question Any proposed missions to explore the black liquid on Titan? Technical challenges? has an interesting line:
ASRG generator was planned as compact energy source but its development didn't end well enough.
which makes me wonder just how well it ended.
The Wikipedia article for the Advanced Stirling radioisotope generator says:
Despite termination of the ASRG flight development contract in 2013, NASA continues a small investment testing by private companies. Flight-ready Stirling-based units are expected by 2028.
which is still so far in the future that it seems speculative.
The downside of the technology versus thermocouple-based heat-to-electricity conversion includes things like added complexity and moving parts, but I assume the upside is a higher efficiency which means for a given power output either a smaller quantity of a given isotope can be used or a less-effective alternate radioisotope can be selected.
The energy conversion process used by an ASRG is about four times more efficient than in previous radioisotope systems to produce a similar amount of power, and allows it to use about one quarter of the plutonium-238 as other similar generators. (emphasis added)
So I'd like to ask; what is the current status of Stirling engine-based radioisotope generator technology? Has the technology fallen by the wayside in lieu of solar panels, thermocouple-based RTGs and kilopower fission reactors, or is this technology coming along nicely and may indeed be ready in 2028, or even sooner?