But 135 million is the number of journeys, the number of unique people taking these journeys will be much much lower. If you take the metro twice every weekday,

But 135 million is the number of journeys, the number of unique people taking these journeys will be much much lower. If you take the metro twice every weekday, that is already around 500 journeys per year for one person, so the number of unique passengers per year would probably be much lower, more like single digit millions. The resulting likelihood is still low, but the chance of something happening over the next couple of years in a place that you are in frequently, thus forever shattering your sense of security in a city that otherwise has a super low crimerate is not that off. You don't have to be killed to be affected in a negative way by something like this after all. Adding to that is the fact that these kind of events have increased in frequency as of late and who knows how it develops in the future, especially if you country just took in huge amounts of syrian refugees without any background checks whatsoever.