We don't know very well how the body works on the higher "interconnected" level of how "X affects Y down the road over time" so preventing it from getting midly ill from a simple flu once a year might end up causing severe unexpected issues down the road.
[EDIT: I see it's getting downvoted a lot: do you really belive that you can just eliminate a completely harmless to young people disease and assume you've done so in complete isolation of all other factors? Nothing is ever done in isolation in the human body, everything is always interconnected and you cannot predict how this will affect the body many years down the line. Don't be suprised if young people will then also start having severe issues all of the sudden that were once reserved only for the crtiticaly ill and very old.]