Agreed, but I will say that it could still be done. Sigourney Weaver is pretty fit and active lately, far more than most people her age, so its not entirely out of the question.
I will say that if they don't do it soon, and I mean VERY soon, it will never happen. Its not just her but Hicks and Bishop (if he was to be included), all actors aging. Two or three more years down the line, it probably has no chance of working vs the slight chance of it working IF they do it right. Big IF.