“Those rabbis who allowed smoking (e.g., Rav Moshe has a teshuva about this) did so because in their days, it was not yet known just how dangerous smoking was, and if they would have known, they would have assured it as well.”
I would be very wary of making definitive statements regarding what Gedolei HaDor “would have said” in a given situation. I would at the very least qualify the statement by adding the word “perhaps”.