Ginsberg will live to be 105. I see people constantly chortling about replacing her as well but if she had any doubts about her health she would have given Obama her resignation. I know she's had cancer a couple of times but she was fine after the first bout in '99 and seemingly fine after the 2nd in '09. She's active, popping up all over the place to give speeches, and has the best care money and power can provide.
At 85 anything could happen, but I don't think she'll be replaced by Trump. At least not in his first term.