I think it’s very likely Thomas retires next year. He might want to enjoy a retirement on the speaking circuit and replace himself with a younger conservative judge. 2019 is the best time to go. Before the controversy of election year and if god forbid Trump loses, he wouldn’t be potentially trapped waiting for another Republican administration.
Barrett is a very conservative Judge and would make a good replacement for Thomas. I don’t think you can count on RBG leaving the court. That woman hates Trump and will find a way to outlive his administration.
Thapar would be a good replacement for Thomas.
Ruthie is 85 now, and not in good health. I don’t know how she lasts until 2024.