I don't see Ginsberg or Breyer retiring willingly as long a non-D is POTUS. If there's a retirement coming then Kennedy (80 and approaching 29 years on the court) is most likely, followed by Thomas (68, but 25.5 years on the court).
However Ginsberg or Breyer could pass away at any time (well, technically any of the SCOTUS justices could), and it's highly unlikely both of them make it to 2024. Ginsberg is 84 and has been on the court for 24 years and as noted has had some major health problems. Breyer is 78 and has been there for 23 years though his Wikipedia page doesn't mention any health problems.