If you were eating a soup from a bowl with 500ml of soup taking 25ml spoonfuls, and the rain replaced the volume that you ate at the same rate as you ate it, how many spoon fulls would it take for the soup to be completely replaced with water? Also, when that happens, would it still be the same soup?
At one point you only have 1 molecule of “soup” left, and a 50/50 chance of getting it so statistically you might never.
My soup I’ve got for lunch today is guesstimated at 40-60% water pre-rain. So it could very well be indistinguishable from rain water long before a one molecule scenario.