When water fountains were segregated, everyone drank from the water fountains. When integration came, businesses tried those water fountains with the little pointed bottom cups. Then folks migrated to the bottles. Fluoridation may have been a minor factor.
Lunch counters/Soda Fountains used to have glasses. Integration started, and some of the storekeepers would throw away the glasses that certain of the customers used. That’s why fast food joint went to paper cups.
Fast food has always used paper or plastic cups. No need to wash glasses, and the only difference between takout / drive-thru and eat-in is bag vs tray (the trays do need to be washed).
A few years ago, under green pressure, McD phased out polystyrene foam coffee cups in favor of waxed paper cups.